Charging 2 lifepo4's in parallel

[robb]

Hi I have a ping and a cammy battery. I am touring for a few days and it would make it simpler to be able to charge both batteries in parallel. I plan to use 2 diodes and charge them as in diagram. Are there any potential problems with the idea?
Cheers Rob

 

[Tiberius]

Robb,

That circuit will work. You could equally put the diodes the other way round in the negative path for the same effect.

A few things to watch, though.

I'm assuming the negative leads are going to the BMS negative charge line (sometimes its blue) and the batteries are not still connected to the vehicle.

Check the diodes will stand the heat. If its a 2 A charger then no problem, but if its a 10 A one, then you could have up to 10 W dissipated in a diode.

With a standard dumb LiFePo charger there is no problem, but some smart chargers for NiMH, etc like to sense the battery voltage before they turn on, and they won't work with diodes in place.

Nick

 

[robb]

Thanks Nick , I will be charging them with the standard 3 to 5 watt ping or cammy charger. They will be disconnected from the motor whilst being charged. I have just bought 2 6 amp diodes from Maplins.http://www.maplin.co.uk/Module.aspx?ModuleNo=46413 Pretty ignorant of electronics, should these do? Hope to make up a lead to connect the 2 batteries to the charger in the next few days.
Rob

 

[robb]

hi just resurrecting thread as I tried charging the two batteries in parallel recently and no charge is going through the diode to one of them , the other is charging fine. Presumably the diode has failed. Logically I do not see why I need the diodes at all. Each battery has a BMS so should not the BMS cut off the charge from the charger when the battery is fully charged and also prevent one battery leaking to the other. Why should I not do away with the diodes and simply connect the charging inputs of each battery together in parallel as below

 

[Spacey]

I've never needed to use diodes.

 

[dogman dan]

Charge twice as fast with two chargers is what I would do. I could see it getting a bit weird, if one battery was balanced nice, and the other wasn't. You'd end up either not balancing one of em, or the balanced one would work it's bms more than normal.

Paralelling the batteries at the cell group level, that makes some sense, just like we do with lipo. Then one bms could run both batteries.

 

[robb]

Charge twice as fast with two chargers is what I would do. I could see it getting a bit weird, if one battery was balanced nice, and the other wasn't. You'd end up either not balancing one of em, or the balanced one would work it's bms more than normal. <<

Logically if one battery charges before the other should not the bms cut it off from the charging circuit leaving the charger to charge the remaining battery until it is also fully charged when it's bms will also cut off. I am quite hazy on electonics so don't want to risk something that might damage the batteries. I do sometimes charge with 2 chargers at home but I am going to stay with a friend and it would be easier to take just the one charger and leave it on overnight to charge both batteries in parallel.
Rob

 

[dogman dan]

I'm fuzzy on how it works too, but the charger also plays a role by tapering the amps down near the end. So two batteries that were paralelled would presumably be at about the same state of discharge. But you could have some issues I think, if the two batteries were dissimilar enough. Like different sizes, or different types of lifepo4. So you could end up putting more wear on one of the bms units? Like one would always charge first? Then the bms on the fast one would run like hell trying to discharge the overcharging cells. Do the bms actually shut off the charge?

One thing I am sure of, two five amp chargers charges faster than one five amp charger.

And I am sure, that if you paralell the two batteries at the balance leads and at the discharge leads, then connect to a bms, it will act like one big battery. Then it would not matter if one was bigger capacity than the other.

 

[passerofgasses]

I have two 48v 20Ah VPower LiFePO4 batteries in parallel in my EVT168 (a 1500w escooter that draws at a peak of nearly 7000w, but I try to keep it at 2000w or less with one eye on the cycleanalyst). On a side note, although purchased at the same time, one battery was delivered unbalanced, probably due to a disconnected connector on the bms. Anyways, I have them connected in parallel but disconnectable from each other using some cheap $1 1500W AC light switches. (I've actually doubled up on the switches [in parallel again] so I figure they act like 3000W switches). I charge the batteries seperately (switches off) using two 5A chargers and the time it takes seems fairly close, like maybe a 5-10% difference (I hope that means they discharge somewhat evenly during use). However, I've forgotten to switch them apart on 2 occasions at least and I've ended up charging both in parallel with 2 chargers at the same time. It seemed to work fine. Unrelated to those 2 charges (a completely different time), my thermistor blew in one of the chargers (an easy fix once I determined the specs of the thermistor) and in the meantime I was able to charge both batteries in parallel using only one charger, which of course takes twice as long as usual. That saved me the time of changing the charger over to the other battery once one was charged.

It seems like the unbalanced battery is having trouble with one or two weak cells now, but that's another story. My vehicle is also very underpowered at 40Ah total, and maybe I'll just let it sit and try to get some ebikes going with the packs instead (my copout plan in case this setup didn't work in the first place), and wait until battery technology (or economics) makes it more feasible.

Sorry if this post seems irrelevant to the issue of diodes and such, but I thought I'd just throw in my experience with the parallel hookup bit.