Parallel Batteries

[scoge]

value your advice on the battery options I need to change on my motorhome.
I have a Autosleeper Worcester/Winchcombe van year 2013 which is on a Mercedes Sprinter base vehicle.
Current set up is one Numax DC25MP 105Ah C20 850A Battery, approx. 3 years old, charged/managed through a Sargent EC500 control unit with a EC480 display panel.
The van has a Sargent PX300 Battery Charger with a 3 stage switching mode.
There is a solar panel on the roof which I estimate to be 100w managed by the Sargent set up which appears to be a standard fitment from new.
The Sargent seems to control everything satisfactorily and I can manage 2-3 nights off grid in the summer before the TV especially tells me low voltage and the power display shows a low voltage on the battery display.
I would like more power to extend my time off grid and especially in the winter when the solar doesn't assist too much and we need to run 12v heater fan. My power needs are modest, I have led lighting, gas fridge and gas heating so just need a boost for phone/tablet charging, modest TV watching and the 12v heater fan for those colder days when we want to be off grid for maybe 5/6 days or so. I do not require an inverter.
So I am hoping the installation of 2 new matching batteries wired in parallel will do the job. I do fancy lithium but I am aware that the Sargent system would probably not be appropriate so the cost of extra chargers/B2B units etc along with the battery cost may be over kill for what I think is only a modest power upgrade requirement. So is the wiring diagram below correct and would 6.0mm Sq cable do the job for all the cabling along with the two 50amp fuses?https://servimg.com/view/20525190/1
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scogeMember

Posts : 20
Joined : 2023-02-04
Location : Stourbridge
Auto-Sleeper : Worcester
Vehicle Year : 2013

 

[LithiumConvert]

Do not change the fuse size.

The charging current will be same - the two batteries will just take twice as long to charge - and twice as long to discharge (which is what you want).

It is not usual to have a fuse on the negative side. The fuse on the positive is essential - as close to the batteries as you can get it.

 

[pappajohn]

And, unless the batteries are side by side and no chance of any wiring touching earth....chafing etc....fit fuses at BOTH ends to the pos wire between batteries.
Why?
If the link wire should touch earth the single fuse will blow BUT the wire is still connected to a live battery at the other end.

 

[scotjimland]

.

 

[LithiumConvert]

Diagram is good, but the OP should stick to his original fuse size. (all 20 amp, if more than one)

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[Raul]

Definetly not 50a fuse on a 6mm2 wire.
The size of the wire is dependable on load and distance. The 6mm2 its ok for short distance, the 10mm2 will be better and will reduce voltage drop, with a 30a fuse.

 

[RogerIvy]

As Raul says, not a 50A fuse on that wire.
But note that two batteries in parallel can “draw” more charge than a single battery.
So I would increase the 20A to 30A (or change to 10mm or 16mm cable).

 

[LithiumConvert]

But note that two batteries in parallel can “draw” more charge than a single battery.

Technically correct, but on motorhome charging systems, the current is almost certainly decided by the charging system, not the batteries.

 

[scoge]

Thanks all for comments.
The batteries will be adjacent to each other.
So I will use 10mm2 cable and change the fuses to 30amp and place fuses next to the batteries.

 

[RogerIvy]

Technically correct, but on motorhome charging systems, the current is almost certainly decided by the charging system, not the batteries.

A split charge system basically puts the leisure battery pack in parallel with the starter battery and the charging system doesn’t manage how much current flows into the battery. The thin wires may then restrict the charge somewhat but something could easily blow.

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[autorouter]

You have to distinguish between two situations. The first is increasing the the power used at any particular time, eg adding an inverter, or lots of higher power devices. The second is keeping more or less the same power but using it for an extended time. The first one requires thicker wires and larger fuses to cope with the increased amps from the new loads. The second one can keep the wiring more or less the same, because the amps will stay much the same, the only difference is that the new battery bank will last for more days off-grid.

It looks to me like the OP wants the second option, ie extended time with the same loads, not a big increase in the loads at any one time. Items like a TV, laptop or tablet will not substantially increase the total load.

As RogerIvy says, the maximum current will probably be reached when the alternator is charging through the split charge relay, and that might increase a bit, although it will probably stay much the same and just take longer to charge.

So I would vote for LithiumConvert 's option of not doing very much in the way of alterations. The battery link wires should be the same thickness as the existing wires. Thicker is OK if you already have some wire and don't want to buy some specially.

Also I can see no reason at all for a fuse in the negative wire, and am surprised that it is there in a manufactured conversion like Autosleeper. A previous user must have added it. If there was an 'overcurrent situation', it would be 50/50 which one would blow first. If the negative one blew, the rest of the circuit would still be 'live', not isolated. I would be tempted to scrap the fuse in the negative wire.

If you want to calculate the wire thickness, the approximate length of the wire is needed. A 5m run would need thicker wire than a 2m run.

 

[PhilandMena]

I too have the Sargent system (EC 500 & PX 300 Charger). Sargent restrict solar panel input to a current no greater than 10 Amps otherwise their PSU can be damaged. ((I guess Autotrail said 100 W when in fact they could have safely said 150 W and possibly higher without any risk to the system) Anyway, I fitted a 125 W German S.P and fed it into a Tracer 10 AMP MPPT controler and just bypassed the Sargent PMW controler. This way the control panel still gives me cab and hab readings and can charge both sets of batteries (Hab =2 x 110 AH LA) from the SP, via the Sargent system.
I soon realised that this was indufficiant for my needs, similar to yours, with fridge lights, TV, 300W Inverter. A fair bit of evening TV watching and very little EHU use so have now added a 250W Panasonic HIT S.P. and gone straight to the Hab battery bank via a Tracer 20 Amp MPPT controler. I have also fitted 2 new Leoch 105 AH Pure Lead Carbon deep cycle AGM batteries. The charging profile for these batteries would be harmed by the Sargent charger as every 30 days the Sargent charger introduces a 15V equaliation charge which is too high for the Leoch batteries, so I have set the chafging profile in both controllers to suit the Leoch battery and the 10 Amp SC also charges the cab battery correctly without over charging the Hab batteries. I tested my set up by going off grid for a month in Germany last month and all went well.
Before making this change I looked at Lithium but concluded the Sargent system and alternator was not suitable for these batteries without a fair bit of alteration and I could get the increased battery life with these newish pure lead carbon boys. We shall see!

 

[scoge]

You have to distinguish between two situations. The first is increasing the the power used at any particular time, eg adding an inverter, or lots of higher power devices. The second is keeping more or less the same power but using it for an extended time. The first one requires thicker wires and larger fuses to cope with the increased amps from the new loads. The second one can keep the wiring more or less the same, because the amps will stay much the same, the only difference is that the new battery bank will last for more days off-grid.

It looks to me like the OP wants the second option, ie extended time with the same loads, not a big increase in the loads at any one time. Items like a TV, laptop or tablet will not substantially increase the total load.

As RogerIvy says, the maximum current will probably be reached when the alternator is charging through the split charge relay, and that might increase a bit, although it will probably stay much the same and just take longer to charge.

So I would vote for LithiumConvert 's option of not doing very much in the way of alterations. The battery link wires should be the same thickness as the existing wires. Thicker is OK if you already have some wire and don't want to buy some specially.

Also I can see no reason at all for a fuse in the negative wire, and am surprised that it is there in a manufactured conversion like Autosleeper. A previous user must have added it. If there was an 'overcurrent situation', it would be 50/50 which one would blow first. If the negative one blew, the rest of the circuit would still be 'live', not isolated. I would be tempted to scrap the fuse in the negative wire.

If you want to calculate the wire thickness, the approximate length of the wire is needed. A 5m run would need thicker wire than a 2m run.

Yes second option, no inverter. I am happy that as my one battery is not in new condition 2 new matched will more than double my capacity off grid.

 

[scoge]

I too have the Sargent system (EC 500 & PX 300 Charger). Sargent restrict solar panel input to a current no greater than 10 Amps otherwise their PSU can be damaged. ((I guess Autotrail said 100 W when in fact they could have safely said 150 W and possibly higher without any risk to the system) Anyway, I fitted a 125 W German S.P and fed it into a Tracer 10 AMP MPPT controler and just bypassed the Sargent PMW controler. This way the control panel still gives me cab and hab readings and can charge both sets of batteries (Hab =2 x 110 AH LA) from the SP, via the Sargent system.
I soon realised that this was indufficiant for my needs, similar to yours, with fridge lights, TV, 300W Inverter. A fair bit of evening TV watching and very little EHU use so have now added a 250W Panasonic HIT S.P. and gone straight to the Hab battery bank via a Tracer 20 Amp MPPT controler. I have also fitted 2 new Leoch 105 AH Pure Lead Carbon deep cycle AGM batteries. The charging profile for these batteries would be harmed by the Sargent charger as every 30 days the Sargent charger introduces a 15V equaliation charge which is too high for the Leoch batteries, so I have set the chafging profile in both controllers to suit the Leoch battery and the 10 Amp SC also charges the cab battery correctly without over charging the Hab batteries. I tested my set up by going off grid for a month in Germany last month and all went well.
Before making this change I looked at Lithium but concluded the Sargent system and alternator was not suitable for these batteries without a fair bit of alteration and I could get the increased battery life with these newish pure lead carbon boys. We shall see!

No inverter for me just wanted to keep my upgrade simple. Winter outings will show me if I need more tweaks.

 

[PhilandMena]

No inverter for me just wanted to keep my upgrade simple. Winter outings will show me if I need more tweaks.

Best of luck going forward. A 100 W solar panel output in the UK over winter is usually a big disappointment.

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