Permanently paralleling two Lifepo packs together

[Lu.Sochr]

Hello everyone,

I would like to ask you about possibility of paralleling two LiFePo packs. I have ordered two new 48V12Ah packs, so they should be exactly the same and new. Every pack has its own BMS with separate charge and discharge connection. I would like to put them in parallel in order to get 48V24Ah. I wonder if there would be any problem if I would parallel discharge leads as well as charge leads. I would like to discharge them at once and charge them at once with one single 48V/5A charger.

I do not plan to use any of this pack separately, I would like to parallel them and let them that way forever.

Would it be possible? Will balancing work fine if there are two separate BMSs???

Thanks in advance for your advices

 

[dnmun]

no you cannot charge them in parallel through the separate BMSs. when charging the two BMSs have to be isolated at the drains to allow the HVC for each BMS to operate.

you can combine them during discharge. you can also tie the two batteries together under one BMS and tie the two packs together in parallel att eh cell level by connecteing them through the sense wires and then tie the B- of each battery together and the B+ together and then the C- of the charger is only one lead and the P- is only one lead.

it would then balance and discharge as one pack. you can use either one or two chargers to charge.

 

[999zip999]

What two lifepo4 packs ? Or disconnect for charging and use two 5a. chargers

 

[docnjoj]

I do this on my wife's trike. I charge 6 parallel Fatpacks with the Bosch charger and the 10 ah Ping with his 2 amp charger. The batteries are separated. Then when both are charged I parallel the leads for the ride. Voltages are similar enough so no transfer of energy occurs and i get a total of about 18 usable a.h. Been working fine for about 2 years now. No BMS on the Fatpacks and they still self balance. Regular BMS on the Ping.
otherDoc

 

[Rassy]

I have two 10AH Ping batteries on one of my trikes. I added a basic dual battery marine switch between them and now it's just a simple flick of the switch to isolate the batteries or parallel the batteries. I always isolate them during charging. Plus, as mentioned above, you can then use two chargers and cut your charge time in half.

 

[skeetab5780]

its seems like it would be fine to parallel them and use 1 bms. why wouldnt it work. just make sure every cell is paralleled together correctly

 

[friendly1uk]

One bms on two cells in parallel? How will it know one from the other. If one cell is not playing ball, the bms will be blind to it. If one cell is doing all the work, you could pass the C rating of that cell. You wouldn't know. It is not a step in the right direction.

Pairing them at different charge levels might be fun. I guess the bms units won't allow the charges to equalise, Or you would need a lamp or something between them to balance the pack voltages in a controlled manner. Something I have not heard of.

I thought a few diodes would allow the charge points to be in parallel. Four at the charge point to keep them apart, and perhaps one at each packs main output so a bms can't see voltage from another pack. I guess diodes are not perfect though. I intended to parallel some bms controlled packs in the future, So My ears are wide open to any info on this subject

 

[dnmun]

i explained already. you cannot charge them in parallel connected through the drains or P- terminal. if you disconnect them at the P- terminal they can both be charged from the same charger.

 

[Rassy]

Just another clarification. With the marine switch I can also select either battery. So before going to the parallel position on the switch (both batteries) I make it a practice to first check the voltage on each separate battery to make sure they are at the same charge level, using the Watt meter that is permanently wired downstream from the switch.

I don't worry about LVC because I don't abuse my batteries and never run them to empty. In a dire circumstance my plan is to just run one battery down at a time at the very end so I don't have to worry about the BMS issues that could crop up in a dual battery setup.

 

[999zip999]

Most batteries have more then one cell in parelle before the bms so hooking two packs to one bms is the same thing. 4p16s or 8p16s - 4p and 8p act like one cell and no you don't get to track every cell on cell level. So 4p is one cell and 8p is one cell in a series string. Each parelle cells charge to 3.65v as one cell or ?

 

[friendly1uk]

Most batteries have more then one cell in parelle before the bms so hooking two packs to one bms is the same thing. 4p16s or 8p16s - 4p and 8p act like one cell and no you don't get to track every cell on cell level. So 4p is one cell and 8p is one cell in a series string. Each parelle cells charge to 3.65v as one cell or ?

Thing is, if your going to divide a cell in to two, why not take advantage of the situation and monitor both half's separately. This is what I have done.

I see many commercial packs use parallel cells and was hopeful the bms used watched each and every cell individually. To be sure I made my own. I couldn't quite think how they had done it. Perhaps it can't be done. I didn't dwell on it.



I will go and dwell over the parallel charging some time. It matters to me, but just being told how it is will never satisfy me, and it is way to much for someone to explain. The fact's give me a good starting point though, and I know the oz890 is well documented so It shouldn't present a problem. I was hopeful it was already laid out in a thread somewhere, but it's no hardship

 

[999zip999]

I want to know what two 48v12ah batteries you are talking about. More info is needed and what are you going to use it on with the amp draw of your controller.

 

[friendly1uk]

I will go and dwell over the parallel charging some time. It matters to me, but just being told how it is will never satisfy me, and it is way to much for someone to explain. The fact's give me a good starting point though, and I know the oz890 is well documented so It shouldn't present a problem. I was hopeful it was already laid out in a thread somewhere, but it's no hardship

Got it. My smart bms is no good for sharing a charger. As dnmun said, No single bms could cut the chargers minus from it's battery, as the battery is in parallel with another who's bms could keep supplying the pair of batteries with the chargers minus.

What I need to do is put another wire between the fets on each board. Then both boards would cut off the charger. It's just an idea though, it needs work.

 

[Lu.Sochr]

Thanks very much for your ideas. I am not sure who is manufacturer of my lifepo packs. I purchased them from our local golden motor vendor, but I think it is not GM packs, because on official GM site, I could not find them. I will attach photo, so maybe you could help me to identify them. I do not want to rewire cells to one bms, because I would have to disassemble them and that would violate warranty. So I would like to parallel them as they are.

 

[dnmun]

if you have that setup then you have to disconnect at the black wire when charging.

you can leave the red wire attached to the controller all the time and put a connector in the black wire so it can be disconnected while charging.

 

[999zip999]

Can he just make a Y connection for the neg. and pos. wires for the bike as a 24ah battery and diconnect the Y and charge as two batteries. Get to charge the packs two at one time for faster charge and hooked togethere for 24ah bike battery. Using the same plastic connetors that are used on the battery and not have to cut the ends on the new packs for better 4mm gold bullets ( the ones I would use ) for warranty stuff.

 

[dnmun]

as i have said several times. they can be combined but they have to be separated at the drains on the BMS.

for this BMS he has he has to open the black wire when charging in parallel. if they connect in a Y the he only has to open one side of the wye.

 

[Lu.Sochr]

Would it be OK, if I would parallel discharge leads (only) when discharging and then parallel charge leads (only) when charging?

 

[Lu.Sochr]

So I have done it!

Everything seems to be OK, although when I charging those two batteries together, charger does not switch green led on. Both batteries are fully charged (sitting on 58,2V) but charger is still running infinite time (no current is flowing). But if I split two packs and charge them separately to full, charger does go to green led and sitting on 58,4V. Do anybody know why charger refuses to go till 58,4V when charging both packs together???

Thanks in advance for your replies...

 

[Lu.Sochr]

Anybody???

What if I would tune POT in the charger to increase stop current value???