LiFePO4 Charging: AC to DC Charger: Multiple batteries?

[oguruma]

I have 2 100AH LiFePO4 batteries. I'd like to wire them in parallel with one in the bed and one under the hood.

I have a Victron AC-DC charger, but it only has one one DC output. Will this work to charge 2 batteries in parallel?

 

[DiploStrat]

The short answer is that two batteries in parallel act as one battery. So your charger only needs one output.

The broader question is the wire distance implicit in your setup.

-- The batteries in parallel should be charged (and discharged) "diagonally." That means that the charger and the loads are wired to the positive of one battery and the negative of the other. This makes the draw on the batteries more even.

-- The paralleling wires between the batteries will need to be big enough to sustain your biggest draw without much voltage drop. So you will need to know what that is and size your wires accordingly. If not, your 100Ah battery becomes effectively smaller.

-- The same wire sizing issue will arise with the Victron - the wires have to be big enough to carry the rated output over the distance without significant voltage drop.

The short suggestion is put them both in the bed - things will be much neater and easier.

 

[Red90]

The batteries will have built in battery management systems, so it will work fine. You do not need to do anything special. You just give a constant 14.4 voltage and the batteries manage the charging internally.

 

[Dave in AZ]

The batteries will have built in battery management systems, so it will work fine. You do not need to do anything special. You just give a constant 14.4 voltage and the batteries manage the charging internally.

Hmm... that's not the case according to about 1000 posts on Will Prowse's DIYsolarforum:

Vehicle Mounted Systems

RV's, Vans, Trucks and More! Typically 12v, and 100-1000w, but the limits are endless!

diysolarforum.com

In fact, a constant 14.4 volts is NOT the optimal charge schedule for most all LFPs, no matter what BMS they have. Each manufacturer specifies voltages for bulk, absorbtion, float, and storage charging, and they are not the same for all LFPs. Some like Renogy have an odd number of cells that makes these voltages significantly different from others. And a good dc to dc charger can be set to these voltages for optimal charging.


Any disparity in cable length or resistance between the batteries and charger, or batteries and load, will cause one to charge faster and discharge faster. Different cable temps between bed and engine bay will cause different resistance also. This makes one battery finish bulk charging and go into float before the other. This will cause balance issues that can significantly decrease battery life. Yes, over time without use they can self balance, but exact same cable lengths is a critical design requirement.

Putting both batteries physically together will remove most all of these issues and let you charge with one charger. Or better yet, buy a 200Ah battery, then you just have one BMS to deal with.

 

[Red90]

Hmm... that's not the case according to about 1000 posts on Will Prowse's DIYsolarforum:

Vehicle Mounted Systems

RV's, Vans, Trucks and More! Typically 12v, and 100-1000w, but the limits are endless!

diysolarforum.com

In fact, a constant 14.4 volts is NOT the optimal charge schedule for most all LFPs, no matter what BMS they have. Each manufacturer specifies voltages for bulk, absorbtion, float, and storage charging, and they are not the same for all LFPs. Some like Renogy have an odd number of cells that makes these voltages significantly different from others. And a good dc to dc charger can be set to these voltages for optimal charging.


Any disparity in cable length or resistance between the batteries and charger, or batteries and load, will cause one to charge faster and discharge faster. Different cable temps between bed and engine bay will cause different resistance also. This makes one battery finish bulk charging and go into float before the other. This will cause balance issues that can significantly decrease battery life. Yes, over time without use they can self balance, but exact same cable lengths is a critical design requirement.

Putting both batteries physically together will remove most all of these issues and let you charge with one charger. Or better yet, buy a 200Ah battery, then you just have one BMS to deal with.

You seriously have no idea what you are talking about. The batteries control the charge and cell balancing internally. It matters not what happens outside.

 

[Dave in AZ]

You seriously have no idea what you are talking about. The batteries control the charge and cell balancing internally. It matters not what happens outside.

Great! You should tell all the manufacturers of LFP battery chargers, and dc to dc chargers--Victron etc-- that they're wrong too, and that their products aren't needed and pointless! Will Prowse has 10 videos or so about setting up parallel LFP battery banks and cable length issues, you should let him know it's all b.s. too!

 

[jonyjoe101]

Once the 2 batteries are connected together the voltages will stabilize to each other, even over distances, the higher voltage battery will charge the lower voltage so the voltage stabilizes.
I've connected DC amp meters between 2 batteries to see what the voltage transfer is and its very minimal to be inconsequential.
During charge and discharge the batteries voltage will stabilize. A constant 14.4 volt charge to lifepo4 is very safe, since the BMS won't activate unless the voltage reaches more than 14.6 volts (max charge for lifepo4) , and since there will be some voltage drop between the charger and batteries, it will probably be lower than 14.4 volts. I actually charge my lifepo4 to 14.4 volts every day with solar, once it reaches that voltage it just trickle charges at about 1 amp.
For high discharge, high amp charging you might want to have the connecting wires of equal lengths for max performance. But for low amp use it won't matter much.
My 220 ah lifepo4 are 4x 55ah batteries connected in parallel, they been running like that for almost 5 years.
My 312ah li-ion battery bank, is 12x 26ah batteries connected in parallel.
My only concern is I wouldnt put a lifepo4 under the hood. Its too hot when the engine is running and the BMS does have a circuit that will disconnect if its too hot or cold. If you live in freezing temps, having a lifepo4 expose to the cold weather will be bad also.

 

[oguruma]

The short answer is that two batteries in parallel act as one battery. So your charger only needs one output.

The broader question is the wire distance implicit in your setup.

-- The batteries in parallel should be charged (and discharged) "diagonally." That means that the charger and the loads are wired to the positive of one battery and the negative of the other. This makes the draw on the batteries more even.

-- The paralleling wires between the batteries will need to be big enough to sustain your biggest draw without much voltage drop. So you will need to know what that is and size your wires accordingly. If not, your 100Ah battery becomes effectively smaller.

-- The same wire sizing issue will arise with the Victron - the wires have to be big enough to carry the rated output over the distance without significant voltage drop.

The short suggestion is put them both in the bed - things will be much neater and easier.

I thought about putting them both in the bed, but the unused space under the hood (some models of F-250 have 2 batteries from the factory) is really agitating my OCD. That said, I do want to mount the battery in the bed right up against the cab, so at least the wire run wouldn't be as long as if I was running it from the tail end of the bed.

 

[DiploStrat]

I thought about putting them both in the bed, but the unused space under the hood (some models of F-250 have 2 batteries from the factory) is really agitating my OCD. That said, I do want to mount the battery in the bed right up against the cab, so at least the wire run wouldn't be as long as if I was running it from the tail end of the bed.

Since we are spending yer money ...

-- Batteries are made up of cells, typically about 3v each. Put 'em together in series, ya get 12v. Need more amps, put groups in parallel. Connecting batteries in series or parallel is not a problem, but, as noted, there are rules and best practices.

-- I would not put a lithium iron battery under the hood. I have enough worries about heat as mine are next to the hot water heater. If you must use the space, stick another starter battery there.

-- If you have not spent money on a pair of 100Ah batteries yet, don't. I would go directly to one of the new 250-300Ah batteries, like the Battle Born "Gamechanger." Much easier to mount/wire and twice the power in not that much larger a space. Also saves you carpentry work inside your camper.

-- If using a B2B, like the Victron, place it as close to the target battery as possible. This reduces the chance of voltage drop, although this is much less of an issue with lithium than it was with lead acid. The run from the starter battery does have to be adequate, however. And should be fused at the starter battery. At the destination battery as well, if the run is over a meter or so.

As always, YMMV. Best wishes.

 

[DirtWhiskey]

I'm about to expand from 2x 100ah Lifepo4 to 4x. I've seen had the two diagrams below recommended for equal total circuit resistance. Gonna give the first one, called "halfway" by Victron, a whirl FWIW:

 

[DaveInDenver]

@DirtWhiskey the normal two battery cross style expanded to four is alright and is the minimum but the Victron topology is best.

The compromise comes down to cost and access. If the layout makes the middle connection difficult to achieve feeding across has benefit over the long branch parallel, which is guaranteed to wear the bank out unevenly and over time end up with wildly unequal SOC.

Also Iota is showing theoretical in their graphic, you may not see quite this much difference in outside vs inside on method 1 while method 2 won't be perfectly equal. Either is acceptable in practice, which generally suggests no more than 15% difference between the best to worst in a bank.

Also remember that both charger and load have to be wired in these ways to really have the best chance to stay balanced.

 

[DirtWhiskey]

@DirtWhiskey the normal two battery cross style expanded to four is alright and is the minimum but the Victron topology is best.

The compromise comes down to cost and access. If the layout makes the middle connection difficult to achieve feeding across has benefit over the long branch parallel, which is guaranteed to wear the bank out unevenly and over time end up with wildly unequal SOC.

Also Iota is showing theoretical in their graphic, you may not see quite this much difference in outside vs inside on method 1 while method 2 won't be perfectly equal. Either is acceptable in practice, which generally suggests no more than 15% difference between the best to worst in a bank.

Also remember that both charger and load have to be wired in these ways to really have the best chance to stay balanced.

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View attachment 849446

@DaveInDenver Hugely helpful and confirms my very uneducated suspicion. I have an uneven draw issue with my two battery setup, although I suspect it may be due to a bad BMS. Could also be because of the crappy OG Northstar wiring setup, which consists of wire nuts and inline common 30 blade fuses. Pure garbage. I'll be adding a DC/DC charger (alternator) and redoing the system with bus bars and appropriate fuses and accounting for future solar. These are all 100ah SOK heated units. First two are two years old and 2nd two are new. Old units have less than 50 total cycles. Any special consideration you recommend for mixing old and new batteries?

Sorry for the thread hijack. Hope it helps the OP even so.

 

[Peter_n_Margaret]

To wire multiple batteries in series, I prefer the "star" method.
The positive from each battery goes to a common connector using identical cable of identical length. Same with the earths. Each battery sees exactly the same resistance this way.

In my view, the only point in using a DC-DC charger is if the voltage drop in the too small cables is excessive. I charge from my alternator directly and did so with AGM batteries and now with 320Ah of prismatic LiFeO4.
Cheers,
Peter
OKA196 motorhome

 

[DirtWhiskey]

To wire multiple batteries in series, I prefer the "star" method.
The positive from each battery goes to a common connector using identical cable of identical length. Same with the earths. Each battery sees exactly the same resistance this way.

In my view, the only point in using a DC-DC charger is if the voltage drop in the too small cables is excessive. I charge from my alternator directly and did so with AGM batteries and now with 320Ah of prismatic LiFeO4.
Cheers,
Peter
OKA196 motorhome

Doesn't the DC DC charger prevent the batteries from accepting too much charge and overloading the alternator and also keeps the charge voltage in the acceptable range?

 

[Peter_n_Margaret]

Charge rate is a combination of the voltage offered and the voltage of the battery being charged (and the chemistry of the battery). The bigger the difference, the higher the rate of charge. As the voltage of the battery being charged increases, the charge rate decreases.
An alternator does not over charge the crank battery. Its output voltage is typically 14.2V which is relatively low and most batteries are happy to float at that.
Cheers,
Peter
OKA196 motorhome