DC/DC convertor: what does it do?

[Louisesjpp]

I'm trying to understand the electrical system of my new-to-us A Class. It has an AGM house battery, solar panels and a vehicle battery. The solar panels do not charge the vehicle battery. I need to make this happen, and I think I need to change the AGM house battery for a lithium. The 100Ah AGM does not fulfil demand, I don't have weight allowance to add another so going 150Ah lithium first then seeing if I need another solar panel seems to be the way forward.

There is a NE287 battery charger / power supply, straightforward. There is a NE325 DC/DC convertor, and I'm not sure exactly what this is doing. Can anyone tell me?

I think I then need something like a Battery Master to make the link between the solar system and the vehicle battery.

 

[Jev88]

Battery Master is a good idea.

A DC/DC is also known as B2B (as far as I know). It pulls extra power from the alternator to charge leisure battery via the engine battery. Eg one earth and one POS to each battery in the simplest form.
Enables fast charging when engine running.

 

[68c]

A DC/DC converter, also known as Buckboost can change one DC voltage to another. If the first voltage is higher than the second the excess voltage is converted to a higher current and vice versa. Of course as in all things there are losses usually in the form of heat.
On a motorhome it is normally used to raise the voltage coming from the Alternator to be high enough to fully charge the leisure battery. Although it will charge the battery more it is at a lower current.
Not needed on most vans with thick enough wiring and heavy enough split charge relay.
Edit. I use one to raise the 12volt van supply to 42volt to charge my E Bike.

 

[Louisesjpp]

Edit. I use one to raise the 12volt van supply to 42volt to charge my E Bike.

This was my next question! Is a DC/DC better than running the inverter and using the bike's mains charger while running?

 

[autorouter]

The NE325 DC-DC Converter charges the leisure battery from the alternator while the engine is running. It is needed in new motorhomes which have a smart alternator and a Euro6 engine, because the smart alternator on its own won't charge the leisure battery very well. Also it is a good upgrade for older motorhomes that have a simple split charge relay for charging the leisure battery.

A split charge relay connects the leisure battery directly to the starter battery when the engine is running, so that the alternator can charge both. When the engine stops, it disconnects them so they are independent batteries.

The DC-DC charger that 68c describes is very different. Its output is designed to charge an ebike battery of about 36V, I think. A DC-DC converter is a bit more efficient than using an inverter to produce mains voltage, then stepping it down to produce the voltage that the battery needs.

If it's only for charging an ebike then a DC-DC charger is best. However you may already have an inverter, or maybe thinking of installing one for other reasons. Then using it for an ebike mains charger is worth thinking about. Especially if you have lots of solar and a good DC-DC charger to refill the batteries.

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[68c]

Deleted. Post above explains it well.

 

[dunnah01]

The DC/DC converter will give you 40amps of charge from the alternator whilst driving so your 150Ah battery would charge in 3/4hrs if totally flat. Well worth having

 

[68c]

Yes, into a half charged lead acid battery perhaps; but as the charge increases so does the effective circuit resistance so you would need an unnaceptably high voltage to maintain that 40Amp charge. We have all seen battery charger ammeter readings drop off as nearing full charge. Unless it is also boosting the voltage supplied by the alternator it will be no better than a good split charge system. That higher charge voltage to cram the maximum in will also result in reduced battery life.

 

[68c]

Yes, into a half charged lead acid battery perhaps; but as the charge increases so does the effective circuit resistance so you would need an unnaceptably high voltage to maintain that 40Amp charge. We have all seen battery charger ammeter readings drop off as nearing full charge. Unless it is also boosting the voltage supplied by the alternator it will be no better than a good split charge system. That higher charge voltage to cram the maximum in will also result in reduced battery life.

 

[dunnah01]

Or charged

Yes, into a half charged lead acid battery perhaps; but as the charge increases so does the effective circuit resistance so you would need an unnaceptably high voltage to maintain that 40Amp charge. We have all seen battery charger ammeter readings drop off as nearing full charge. Unless it is also boosting the voltage supplied by the alternator it will be no better than a good split charge system. That higher charge voltage to cram the maximum in will also result in reduced battery life.

Or charged enough after a good drive rather than just charged a bit

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[autorouter]

Yes, into a half charged lead acid battery perhaps; but as the charge increases so does the effective circuit resistance so you would need an unnaceptably high voltage to maintain that 40Amp charge. We have all seen battery charger ammeter readings drop off as nearing full charge.

The point is, a DC-DC charger (B2B) keeps up the constant bulk charge current of 40A, with the voltage gradually rising, until it reaches the absorption voltage of about 14.4V. At that point the battery is about 80% charged.

When it reaches the absorption voltage is flips to constant voltage mode, and the amps gradually falls as you say. But that's only in the final 20% of the charging. The absorption voltage is designed to be acceptable for charging the battery. It is slightly different for the different battery chemistries.

The charging amps depends on which version of the NE325 you have. One does 25A, the other does 40A. I think if it's a single 100Ah AGM then it should be the 25A version. If it's the 40A version then I agree, that's too high for that battery. However it will be fine for a lithium battery which can take a much higher charging current without a problem

 

[AshVanBitz]

A Battery to Battery (DC-DC / B2B / Buckboost) charger is connected to the engine battery and the leisure battery (it's in the name). It is not connected directly to the alternator as you will need different equipment for this (Alternator to Battery (A2B) charger - typically used more in the marine market).

The B2B is connected to the positive of the engine battery and positive of the leisure (and a ground of course). With the new smart alternators, when the engine battery is fully charged it simply switches off so no further power can transfer into the leisure battery(s). This is why all new vehicles should have some form of B2B installed at the factory. The B2B manipulates the alternator into believing that the engine battery requires more power and then the leisure battery steals it. The B2B only works when the engine is running and the engine battery is near to full as it understands that it is the primary and doesn't want to deplete it (preventing you from starting the vehicle). The way it works is that when the B2B is running it pulls power out of the engine battery and starts transferring the power into the leisure battery(s). The alternator sees the high current draw and immediately starts replenishing the engine battery at the rate of what the B2B is pulling. We generally do not install a B2B that is over 50% of the alternators capability. Once the leisure battery(s) is fully charged the B2B will go into a sleep mode (it will still be keeping at an eye on the leisure battery) and then in turn the alternator will also go to sleep. If at any stage there after, either of the battery(s) need charging, the relevant equipment would wake up and start the process again.

The charge rate can vary from model to model and it also depends on the charging profile you have selected (based on the type of leisure battery you have fitted). Lithium for example can receive a much high charge than a standard lead acid battery so the B2B will know this and transfer to appropriate amount. You also need to see how your fridge is wired in. Some vehicles have a separate feed going to the fridge and some run everything through the leisure battery. If this is the case then the fridge can steal anything from around 10-15A when the engine is running and this is only to maintain the temperature that it started of at. You may need to factor this is when you look at how much power the leisure batteries are actually receiving. For example, Carthago's use the Schaudt 25A booster. It may seem to be a good option on paper but when you actually factor in everything and take an accurate reading, typically you will only see around 6-10A finding its way into the batteries. Perfect for standard batteries but not if you have upgraded and want to recharge quickly.

This was my next question! Is a DC/DC better than running the inverter and using the bike's mains charger while running?

Use the inverter to charge the bike batteries and the B2B will keep the leisure batteries maintained whilst you are doing it. When you arrive at your destination you hopefully will have fully charge bike batteries as well as the leisure battery(s).

 

[Ridgeway]

A Battery to Battery (DC-DC / B2B / Buckboost) charger is connected to the engine battery and the leisure battery (it's in the name). It is not connected directly to the alternator as you will need different equipment for this (Alternator to Battery (A2B) charger - typically used more in the marine market).

The B2B is connected to the positive of the engine battery and positive of the leisure (and a ground of course). With the new smart alternators, when the engine battery is fully charged it simply switches off so no further power can transfer into the leisure battery(s). This is why all new vehicles should have some form of B2B installed at the factory. The B2B manipulates the alternator into believing that the engine battery requires more power and then the leisure battery steals it. The B2B only works when the engine is running and the engine battery is near to full as it understands that it is the primary and doesn't want to deplete it (preventing you from starting the vehicle). The way it works is that when the B2B is running it pulls power out of the engine battery and starts transferring the power into the leisure battery(s). The alternator sees the high current draw and immediately starts replenishing the engine battery at the rate of what the B2B is pulling. We generally do not install a B2B that is over 50% of the alternators capability. Once the leisure battery(s) is fully charged the B2B will go into a sleep mode (it will still be keeping at an eye on the leisure battery) and then in turn the alternator will also go to sleep. If at any stage there after, either of the battery(s) need charging, the relevant equipment would wake up and start the process again.

The charge rate can vary from model to model and it also depends on the charging profile you have selected (based on the type of leisure battery you have fitted). Lithium for example can receive a much high charge than a standard lead acid battery so the B2B will know this and transfer to appropriate amount. You also need to see how your fridge is wired in. Some vehicles have a separate feed going to the fridge and some run everything through the leisure battery. If this is the case then the fridge can steal anything from around 10-15A when the engine is running and this is only to maintain the temperature that it started of at. You may need to factor this is when you look at how much power the leisure batteries are actually receiving. For example, Carthago's use the Schaudt 25A booster. It may seem to be a good option on paper but when you actually factor in everything and take an accurate reading, typically you will only see around 6-10A finding its way into the batteries. Perfect for standard batteries but not if you have upgraded and want to recharge quickly.

Nice explanation, learnt quite a few things from that

 

[Louisesjpp]

There's a lot of excellent information here, thank you all

So, my understanding of it: If I swap out my 100Ah AGM for a 150Ah Lipo4, I need to make sure that my DC/DC is 40A and not 24A (I don't know yet) and add a Battery Master to override the Euro6 monitoring. Having done that, I'm probably OK running the inverter while driving to charge the E-bikes but it may be best to leave the fridge on gas while doing so.

Does this seem practical and sensible?

An option is charge the e-bikes from a DC/DC specific for the purpose, but this means more research to find out how and more cost/weight for the kit.

 

[AshVanBitz]

There's a lot of excellent information here, thank you all

So, my understanding of it: If I swap out my 100Ah AGM for a 150Ah Lipo4, I need to make sure that my DC/DC is 40A and not 24A (I don't know yet) and add a Battery Master to override the Euro6 monitoring. Having done that, I'm probably OK running the inverter while driving to charge the E-bikes but it may be best to leave the fridge on gas while doing so.

Does this seem practical and sensible?

An option is charge the e-bikes from a DC/DC specific for the purpose, but this means more research to find out how and more cost/weight for the kit.

Sounds good to me. One more thing to check (and you may have already covered this) and that is if you have a solar panel already installed, make sure it has an MPPT regulator with a lithium profile

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[Louisesjpp]

Sounds good to me. One more thing to check (and you may have already covered this) and that is if you have a solar panel already installed, make sure it has an MPPT regulator with a lithium profile

Hmm... It's a Vechline LM Series Solar Power Intelligent PV Controller and the manual doesn't mention battery type.

 

[Louisesjpp]

And I forgot to add in previous postings, will the Battery Master take care of a connection between the PV panels and the cranking battery to keep it charged while the van is parked up for long periods? At present, the battery discharges over a few weeks, think the alarm must be a greedy consumer.

 

[stevewagner]

I wouldn’t run the fridge on gas while driving.