Split Charge Relay current / amps output

[AntonLaw]

Hey guys, I have a motorhome van that I converted 2015 plate citeron relay L1H1 I'm pretty sure it has a non smart alternator that can output 150amps.

I have a number of s lead acid lesuire batteries wired in parralel that I was planing on fitting a split charge relay to, my question is if I decide to go for one of these 140amp split charge voltage sensing relays and I wire it with thick enough gauge wire to carry say 100amps at 12v, would I expect to see on or near 100amps going through the cable if I put an amp clamp or a shunt on it? If not then why and what would I expect to see? I should mention it's about a 1. 5 meter run between both the lesuire batteries and the starter battery, and let's say my lesuire batteries are down to around 70% full.

I can't seem to find a straight answer online as I think I was given some wrong advic just to buy one of these new dc to dc chargers and be done with it.

Thanks

 

[Lenny HB]

If you want a decent charge rate forget about a split charge relay, you will only get a high charge rate for the first 15 or 20 minutes once the battery voltage starts to rise and the internal resistance increases the charge rate will drop off. Also voltage sensing relays are not suitable if you have solar charging.

Far better to fit a B2B (DC to DC charger) this will control the charge to the batteries. You need to check what the max charge rate is for your batteries normally between 25 to 50% of the capacity depending on the battery.
With a 150 amp alternator I don't think I would want to go above 80 amps.

 

[AntonLaw]

If you want a decent charge rate forget about a split charge relay, you will only get a high charge rate for the first 15 or 20 minutes once the battery voltage starts to rise and the internal resistance increases the charge rate will drop off. Also voltage sensing relays are not suitable if you have solar charging.

Far better to fit a B2B (DC to DC charger) this will control the charge to the batteries. You need to check what the max charge rate is for your batteries normally between 25 to 50% of the capacity depending on the battery.
With a 150 amp alternator I don't think I would want to go above 80 amps.

Thanks for the reply Lenny!

Going on your first point just because I want to understand it all, lead acid I've heard the last 20 percent capacity is the hardest / longest to charge, is this what you are reffering to? With a split charge relay you could go to 50% to 80% fairly quickly (15 to 20mins like you suggested) and the last 20 percent would take a good bit longer? Wouldn't a dc to dc charger be the same as it only outputting the same amount of amps? Maybe I'm not understanding the internal resistance factor you mentioned?

I also didn't relize you couldn't use a split charge relay with solar? Why is this as I've alredy setup my dad's van on this way? Does the voltage form the solar cause the lesuire battery's to spike this flipping the split charge relay to charge the starter battery instead of just charging the lesuire battery's?

Thanks

 

[AntonLaw]

Just so it's maybe a bit more clearer, a dc to dc charger solves the smart alternator problem which I don't have, but it does not charge any quicker than somthing like say a split charge relay rated at 140amp using the correct gauge of cable. The DC to DC charger bacially smooths out the voltage it can't charge the battery any quicker form my understanding.

Example:First 15 / 20 mins
Split charge relay @ 13/14v & 100amps
DC to DC charger @ 13/14v & 100amps

After 15/20mins if the internal resistance of the battery is greater I don't see how both the dc to dc or the standerd relay could charge it any quicker, they battery can only take what it can regardless of what is charging it. I totally get the issue dc to dc charger solve for smarter alternator vans but for me I think I would be throwing away money when a simple relay at probally 1/6 or 1/7th the cost and that's presuming you can find one that cna do over 80amps.

 

[bigtwin]

After 15/20mins if the internal resistance of the battery is greater I don't see how both the dc to dc or the standerd relay could charge it any quicker, they battery can only take what it can regardless of what is charging it.

It can’t. Your understanding is correct.

Ian

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[Lenny HB]

I didn't say you can't use a split charge relay with solar, I said you can't use a voltage sensing split charge relay with solar. But I forgot to say if the starter battery is being charged buy solar as well.

A split charge relay is just a relay the current rating is the amount of current the relay can pass not what it will pass. On a typical system with decent wiring the charge will start at 20 to 25 amps and soon drop to around 10 to 12 amps so to put say 100 ah back into the batteries it is going to take 8 to 9 hours driving.

A B2B acts is a multi-stage charger that acts like a good mains charger with bulk, absorption & float phases & controls the voltage at the battery for optimum charging.

This may help.

B2B, Battery to Battery chargers : Fast, Safe Motorhome Alternator charging

Euro 6 compliant Battery to Battery chargers for modern Motorhomes. Faster Habitation Battery charging that works in harmony with the existing wiring. Does not degrade the battery. Finally a solution to that can make a Hymer fully optimised for AGM batteries.

www.aandncaravanservices.co.uk

 

[Hoovie]

I didn't say you can't use a split charge relay with solar, I said you can't use a voltage sensing split charge relay with solar. But I forgot to say if the starter battery is being charged buy solar as well.

A split charge relay is just a relay the current rating is the amount of current the relay can pass not what it will pass. On a typical system with decent wiring the charge will start at 20 to 25 amps and soon drop to around 10 to 12 amps so to put say 100 ah back into the batteries it is going to take 8 to 9 hours driving.

A B2B acts is a multi-stage charger that acts like a good mains charger with bulk, absorption & float phases & controls the voltage at the battery for optimum charging.

This may help.

B2B, Battery to Battery chargers : Fast, Safe Motorhome Alternator charging

Euro 6 compliant Battery to Battery chargers for modern Motorhomes. Faster Habitation Battery charging that works in harmony with the existing wiring. Does not degrade the battery. Finally a solution to that can make a Hymer fully optimised for AGM batteries.

www.aandncaravanservices.co.uk

I'm not sure about your assertion that you cannot use a VSR if you have solar.
Actually I would say that is just incorrect and not sure why you say that. The combination works fine and is a very common occurrence on many self-builds. I have both used and installed for others that kind of setup and not have any adverse problems.
It does in fact have advantages over a switched relay in that it eliminates the need for add-on devices such as a battery master or AMT-12, or a "Duo" type solar controller.

 

[Lenny HB]

I'm not sure about your assertion that you cannot use a VSR if you have solar.
Actually I would say that is just incorrect and not sure why you say that. The combination works fine and is a very common occurrence on many self-builds.
It does in fact have advantages over a switched relay in that it eliminates the need for add-on devices such as a battery master or AMT-12, or a "Duo" type solar controller.

The problem that often occurs if the starter battery is charged by solar (corrected myself in my second post). As the starter battery voltage rises it turns on the split charge relay so you can fairly easily flatten the starter battery if the hab battery is not fully charged.

 

[AntonLaw]

The problem that often occurs if the starter battery is charged by solar (corrected myself in my second post). As the starter battery voltage rises it turns on the split charge relay so you can fairly easily flatten the starter battery if the hab battery is not fully charged.

A switched relay on ignition between the split charger and the battery would be a good work around for this, but I digress....

Going back to your reply there Lenny, I know you are saying a typical system, but if I have thick enough gauge cable to carry say 150 amps, the alternator spits out a max of 150 amps, let's account for losses etc, if the battery is low enough there is nothing stopping say 80 to 100 amps flowing through the cables and the split charge relay? Meaning it would charge the batteries (lead acid lesuire) faster than a dc to dc (a 100 amp dc to dc cost a whole lot more and a vsr)

Also the DC to DC charger will be taking the same thing in that the relay would be taking in, power form the alternator /starter battery. The only thing the DC to DC does different is boosts or drops the voltage, so it shouldn't not be able to charge any quicker, it can create any more amps than what the alternator / lesuire battery is throwing out?

 

[Hoovie]

The problem that often occurs if the starter battery is charged by solar (corrected myself in my second post). As the starter battery voltage rises it turns on the split charge relay so you can fairly easily flatten the starter battery if the hab battery is not fully charged.

that kind of scenario is not going to happen though unless for some odd reason there is a solar controller on the starter battery itself - whic would be a strange thing to do in itself in a "proper" setup.

the other thing is that even if you did have the above config, the starter battery will not be flattened as a VSR has two thresholds - one to activate and one to deactivate. Activation is around 13.8V, so in the above scenario, the starter battery would be pretty decently charged to allow the VSR to turn on. And the turn-off threshold is around 12.8V, so as soon as the voltage goes below that, the VSR will turn off, so the Starter Battery will never flatten as the VSR will disconnect. it will never even get very low.

VSRs are designed to only be active when there is charging occuring e.g. voltages are ABOVE battery resting voltages at full charge and as soon as a voltage is downat a fully charged resting voltage, the VSR is off. (They don't actually "know" the battery is full, it is the voltage parameters are designed to have that kind of effect).

the time when a VSR is absolutely not appropriate is when you have Lithium, as the resting voltage of Lithium is much greater than the VSR disconnect voltage, and the potential issue there is that the VSR turns on when a charge is present, and never gos off ūntil the Lithium would be down to, oh, 10-15% SOC. (and that still wouldn't affect the starter battery other then keeping it connected to the Leisure battery all the tīme).
what is possible in that situation is tō use a Cyrix-ct Li, which is in effect a semi-intellgent VSR, but with activation thresholds suitable fōr Lithium.


you say the problem often occurs? I've never seen this problem due to a VSR being īnstalled.

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[Lenny HB]

that kind of scenario is not going to happen though unless for some odd reason there is a solar controller on the starter battery itself - whic would be a strange thing to do in itself in a "proper" setup.

Quite normal to have a dual output solar controller charging the starter battery.

you say the problem often occurs? I've never seen this problem due to a VSR being īnstalled.

Several Funsters have had this problem.

 

[Hoovie]

Quite normal to have a dual output solar controller charging the starter battery.

Ī thought in your update you meant a dedicated solar cōntroller on the starter.
anyways, a Duo solar controller is not a sensible setup if you have a VSR. it is pointless.

Several Funsters have had this problem.

well, we will just have to disagree about a VSRs suitablity their setups must have a problem - or possibly faulty VSRs. A correctly working VSR will absolutely not cause the issue you describe for the reasons I gave.

have a think about it and tell me how a VSR can cause a battery to drain if it disconnects when the voltage drops below 12.8 V.

 

[autorouter]

if I have thick enough gauge cable to carry say 150 amps, the alternator spits out a max of 150 amps, let's account for losses etc, if the battery is low enough there is nothing stopping say 80 to 100 amps flowing through the cables and the split charge relay? Meaning it would charge the batteries (lead acid lesuire) faster than a dc to dc (a 100 amp dc to dc cost a whole lot more and a vsr)

The other factor is the alternator voltage. The alternator is designed to charge a single starter battery, so its voltage is limited to ensure that the starter battery isn't overcharged. A split charge relay just splits that charge between the starter battery and all the other leisure batteries. There will probably be some increase in the amps output, but not very much.

A B2B takes power from the alternator in the same way that the lights, fan, wipers etc take power from the alternator. It has internal electronics to output the voltage and amps that the leisure batteries need. It monitors the voltage and amps into the batteries, and gives them exactly what they need. Yes, it will have a maximum amps output limit, but it will ensure that the full amps output is sent to the batteries for as long as it takes to reach the absorption voltage, ie about 80% charged. Then it will set a constant voltage and let the amps drop until the battery is full.

Anyway, you can check all this with a DC clamp meter, as you say. If you find that the split charge relay is not pushing as much charge into the batteries as you would like, you can consider a B2B.

 

[AntonLaw]

The other factor is the alternator voltage. The alternator is designed to charge a single starter battery, so its voltage is limited to ensure that the starter battery isn't overcharged. A split charge relay just splits that charge between the starter battery and all the other leisure batteries. There will probably be some increase in the amps output, but not very much.

A B2B takes power from the alternator in the same way that the lights, fan, wipers etc take power from the alternator. It has internal electronics to output the voltage and amps that the leisure batteries need. It monitors the voltage and amps into the batteries, and gives them exactly what they need. Yes, it will have a maximum amps output limit, but it will ensure that the full amps output is sent to the batteries for as long as it takes to reach the absorption voltage, ie about 80% charged. Then it will set a constant voltage and let the amps drop until the battery is full.

Anyway, you can check all this with a DC clamp meter, as you say. If you find that the split charge relay is not pushing as much charge into the batteries as you would like, you can consider a B2B.

Thanks for the reply, the thing is it doesn't matter if you are charging 1 or 5 lead acid batteries, the voltage remains the same. For the regulator part you mention, a lead acid will only take as many amps as it can, you can't push amps, they are pulled. The only way to over charge a lead acid is to supply it with a higher voltage than it was design for so. Over 14v say, which older alternators like the one in my van never go over. So to say it splits the charge between the starter and the lesuire I don't think is quite right, if the starter is full, current should flow over it and into the lesuire batteries, or it may be pulled from the starter battery which would then be replenished anyway by the alternator.

Again I totally get b2b chargers and whne they should be used, but they can't output more amps than they get from the battery, they can boost or step down the voltage which is advantageous to other battery chemistries but no so much with lead acid that can be charged at a constant 14v, even that last 20% of the lead acids capacity. There are 2 mains stages of charging lead acid bulk and absorption, on bulk we are supplying 14v and all the amps the battery is asking for, on absorption we are still providing 14v and all the amps the battery is asking for, the only difference being during the absorption phase the battery ask for allot less amps as the absorbtion phase of charge that last 20 percent of a lead acid by design is meant to be slow, nothing can speed that up not even a dc to dc charger. The battery is self managed and will only ever pull the amps it needs so a split charge relay will both bulk charge to 80% and also charge the last 20% just like a dc to dc charger would, and if the dc to dc charger is supplying more amps (more so during the bulk phase) than the dc to dc charger or will almost certainly charge the battery to 80 percent quicker.

 

[Googlebot]

I had good thick cables in mine and a basic split charge relay, even with the leisure battery flat it never went above 20amps.

Split charge relays are rubbish, that’s why most people fit a B2B charger.

Try it without and see what happens, then it’s easy to put one in.

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[AdrianChen]

Thanks for the reply, the thing is it doesn't matter if you are charging 1 or 5 lead acid batteries, the voltage remains the same. For the regulator part you mention, a lead acid will only take as many amps as it can, you can't push amps, they are pulled. The only way to over charge a lead acid is to supply it with a higher voltage than it was design for so. Over 14v say, which older alternators like the one in my van never go over. So to say it splits the charge between the starter and the lesuire I don't think is quite right, if the starter is full, current should flow over it and into the lesuire batteries, or it may be pulled from the starter battery which would then be replenished anyway by the alternator.

Again I totally get b2b chargers and whne they should be used, but they can't output more amps than they get from the battery, they can boost or step down the voltage which is advantageous to other battery chemistries but no so much with lead acid that can be charged at a constant 14v, even that last 20% of the lead acids capacity. There are 2 mains stages of charging lead acid bulk and absorption, on bulk we are supplying 14v and all the amps the battery is asking for, on absorption we are still providing 14v and all the amps the battery is asking for, the only difference being during the absorption phase the battery ask for allot less amps as the absorbtion phase of charge that last 20 percent of a lead acid by design is meant to be slow, nothing can speed that up not even a dc to dc charger. The battery is self managed and will only ever pull the amps it needs so a split charge relay will both bulk charge to 80% and also charge the last 20% just like a dc to dc charger would, and if the dc to dc charger is supplying more amps (more so during the bulk phase) than the dc to dc charger or will almost certainly charge the battery to 80 percent quicker.

I suppose I would have some concern about the effect on the leisure batteries if you are doing long journeys with the alternator fixed on its 14.1/14.2v output if there is no drain at all on them, a B2B will drop to a float charge at some point which an alternator wont, especially so if they are AGM. Also the output may be lower than expected if the alternator has some inbuilt temperature protection which can be that the output voltage is dropped, so that the battery accepts less current thereby giving the Alt time to cool - I guess it would do the same in the identical situation even with a B2B in circuit, but I wouldnt count on the Alt being able to run at full tilt for long!

 

[autorouter]

There are 2 mains stages of charging lead acid bulk and absorption, on bulk we are supplying 14v and all the amps the battery is asking for, on absorption we are still providing 14v and all the amps the battery is asking for, the only difference being during the absorption phase the battery ask for allot less amps as the absorbtion phase of charge that last 20 percent of a lead acid by design is meant to be slow, nothing can speed that up not even a dc to dc charger.

That's not how a 3-stage charger works, what you are describing is a fixed constant voltage charger.

The bulk stage is constant-current mode. The current is fixed at the maximum amps output of the charger, and the voltage is whatever the battery decides, less than the absorption voltage if the battery is not fully charged. The charger monitors the voltage and current continuously, as the battery voltage gradually rises to the absorption voltage. When the voltage reaches the absorption voltage, the bulk stage ends, and it flips to the absorption stage.

The absorption stage is constant voltage mode. The charger maintains the voltage at the absorption voltage while monitoring the current, which will gradually fall as the battery completes its charging. When the current has dropped to a predetermined amps value, typically 1/10 of the charger maximum amps, then the absorption stage ends and the charger decides the battery is 100% full. It then flips to the float stage, to avoid overcharging.

The float stage is also constant voltage mode. The voltage is less than the absorption voltage, typically 13.2v to 13.6V, instead of the absorption voltage of 14.4V to 14.8V. This voltage is just enough to keep the battery at 100% and to compensate for self-discharge, and the battery can be left on float charge indefinitely without overcharging.

The exact absorption and float voltages depend on the battery chamistry.

More info here. It's known as IUoU charging, I = Bulk, Uo = Absorption, U = Float.

IUoU battery charging - Wikipedia

en.wikipedia.org

 

[DBK]

Thanks for the reply, the thing is it doesn't matter if you are charging 1 or 5 lead acid batteries, the voltage remains the same. For the regulator part you mention, a lead acid will only take as many amps as it can, you can't push amps, they are pulled. The only way to over charge a lead acid is to supply it with a higher voltage than it was design for so. Over 14v say, which older alternators like the one in my van never go over. So to say it splits the charge between the starter and the lesuire I don't think is quite right, if the starter is full, current should flow over it and into the lesuire batteries, or it may be pulled from the starter battery which would then be replenished anyway by the alternator.

Again I totally get b2b chargers and whne they should be used, but they can't output more amps than they get from the battery, they can boost or step down the voltage which is advantageous to other battery chemistries but no so much with lead acid that can be charged at a constant 14v, even that last 20% of the lead acids capacity. There are 2 mains stages of charging lead acid bulk and absorption, on bulk we are supplying 14v and all the amps the battery is asking for, on absorption we are still providing 14v and all the amps the battery is asking for, the only difference being during the absorption phase the battery ask for allot less amps as the absorbtion phase of charge that last 20 percent of a lead acid by design is meant to be slow, nothing can speed that up not even a dc to dc charger. The battery is self managed and will only ever pull the amps it needs so a split charge relay will both bulk charge to 80% and also charge the last 20% just like a dc to dc charger would, and if the dc to dc charger is supplying more amps (more so during the bulk phase) than the dc to dc charger or will almost certainly charge the battery to 80 percent quicker.

I think you mis-understand all this on many levels but I won't add to what's already been written and instead give you my experience.

Our MH came with an up-rated 150 amp alternator, it was an extra I asked for.

During the first years with the MH I discovered it could take 4 or 5 hours of driving to recharge the batteries after they had been discharged to 50% - measured by their voltage off load being down to 12 volts.

I then fitted a B2B and it now fully charges the batteries in about an hour of driving.

It does this because unlike the split charge system, where the charging voltage would typically fall to around 13.7/13.8 volts after just a short amount of driving, the B2B holds the charging voltage at 14.8 volts then drops it to 13.8 once the battery is almost charged. Charging currents with the split charge system would peak around 25 amps at the start then fall into single figures.

 

[Pausim]

Are you always going to use basic lead acid batteries? A B2B would future proof you for AGM, Gel or LiFePO4 all of which need more precise charging if you are going to get the best out of them.

 

[Hoovie]

......
It does this because unlike the split charge system, where the charging voltage would typically fall to around 13.7/13.8 volts after just a short amount of driving, the B2B holds the charging voltage at 14.8 volts then drops it to 13.8 once the battery is almost charged. Charging currents with the split charge system would peak around 25 amps at the start then fall into single figures.

I don't really see (or would want to see!) 14.8V out of my B2B (did you really mean 14.8V?), but I agree with the principle of what you are saying above regarding the voltages and why a B2B canbe a significant improvement on a basic relay which is of course just an electronic switch, where the Leisure Battery will, at best, see a voltage just a little less than the Alternator will put out, and is often lower than the required charging voltage.

I thought I would grab a shot of a chart illustrating this....

Screenshot 2023-07-26 at 18-47-05 Monty - VRM Portal by David, on Flickr
The Top Blue line is the Starter Battery Voltage. The Orange line on that chart is the current the B2B set is putting into the Leisure Bank (varies between 90A, 60A and 30A depending on which B2B/s are active (which is determined by input voltage programmatically).
The Bottom Blue line is the Leisure Battery Voltage. Batteries are in need of a good charge, so the B2Bs are in CC (Constant Current) BULK mode, rather than the CV (Constant Voltage) ABSORPTION mode, where the voltage will be at the programmed 14.4V, but even in CC mode, you can see the charge voltage at the Leisure Battery is notably higher than the Starter Battery.
If this were just a relay, the charging would be nowhere close as efficient.

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[DBK]

I don't really see (or would want to see!) 14.8V out of my B2B

You're right, my memory is playing up! My Sterling B2B does 14.4 volts for the sealed LA batteries.

Thanks for spotting it.