Parallel combining packs of different volts?

[Jay64]

Ok, this probably seems like a really dumb question to most, but I gotta ask it. If I have two packs of different volts and I combine them in parallel, what is the resulting V? Would it be the number of the lowest pack volts or would it be somewhere in between? If I have a 48v 10ah pack and combine it with a 36v 10ah pack, do I wind up with a 36v 20ah pack or something like a 42v 20ah pack?

Also since I am already asking, can I combine them in series? Would I be able to make a 72v 10ah pack by connecting these in series? The reason that I have to ask this one is because every time I hear about someone connecting packs in series, they are always the same v. So I am wondering if it is bad to combine two packs of different volts in series.

 

[ort5]

Hey Jay,

Never put packs of differing voltages in parallel. They will try to come to an equilibrium voltage instantly. This means lots of power out of the higher V pack into the lower V pack. Bad news.... You should also always put packs of the same voltage in parallel only when they have the same state of charge (for the same reason).

You can put packs of different voltages in series, but they should be of similar ah capacity and discharge rating (and ideally of the same cell chemistry). This is because all of the cells in a series pack will be required to source the same current. If you put a 10ah cell in series with a 3ah cell, this could be bad news. Plus, the 3ah cell would be discharged much sooner than the 10ah cell and lead to possible cell reversal.

This all applies for bare batteries. If you're using something with a BMS, the above still holds true....but you need to be wary of exceeding the BMS limits for voltage and current.

Regards,
Dave

 

[Jay64]

Ok, that's pretty much what I thought, but just figured I would make sure. As you know I have the 48v10ah lifebatt pack and I am purchasing a bunch of dewalt packs. Eventually I could take the dewalt packs apart and build a 48v?s pack, but I wanted to try to keep them intact for as long as possible in my learning/testing phase. I just thought that it would be cool to hook them up with the 48v10ah pack since I have it.
If I do make a 48v10ah a123 pack, it would have a different discharge rating then the lifebatt pack. could I hook these together if I set up my controller draw to the lower rating of the lifebatt and base my usage/recharging rate on the lower pack?

 

[nomad85]

I wouldn't. Just make a bigger A123 pack.(money doenst seem to be an issue for you) I wouldnt want to risk messing up 2 expensive batteries. If you do connect them in parrallel, Use a diode to prevent current passing through them. I've seen them at e-bikes.com

 

[ort5]

If I do make a 48v10ah a123 pack, it would have a different discharge rating then the lifebatt pack. could I hook these together if I set up my controller draw to the lower rating of the lifebatt and base my usage/recharging rate on the lower pack?

I would still be concerned about this, but it seems technically possible. There would be a fair amount of money involved in that pack!

 

[Jay64]

Well, money is an issue with me. I am being sponsored for a lot of the stuff I am buying at the moment, but part of that sponsorship is showing where each dollar goes. I do have a budget. I can't just go out and waste the money. And I do need to figure out a way to make use of the lifebatt pack. It would be hard for me to justify buying another 48v10ah beyond what I already have if I haven't used the first 48v 10ah pack I got. But I also can't afford to be going and blowing up a couple of LiFe packs either. Right now I have enough dewalt packs to make a 70 cell pack. I think that should make me a 48v 9.2ah pack. I am interested in possibly running them as 36v 16ah if I put all 7 in parallel. I was thinking that I eventually would tie the packs together somehow.

 

[fechter]

Since the chemistry is the same, you could put A123 cells in parallel with LifeBatt cells in theory.

You would need to parallel each cell group separately, then put the groups in series.

 

[Rassy]

ort5 said

Never put packs of differing voltages in parallel. They will try to come to an equilibrium voltage instantly

I would add to this, even if the batteries are the same, always check the voltage of each before connecting them in parallel. I sometimes hook my two 48V 20AH LiFePO4 in parallel for my trike (bypassing the BMS), but first either charge or discharge one of them until the resting voltages are within 0.2 volts of each other.

 

[monster]

if in parallel the voltages must be matched, if in series the AH capasity must be matched. chemistries should always be the same.

having said this, i do stretch the rules a little bit myself. i mix Nicd and Nimh in series and parallel with very similar AH capacities. i assume the two chemistries are interchangeable. i often parallel packs with different state of charge (but same nominal voltage and capacity). i assume the full pack just partially charges the empty one.

 

[docnjoj]

Yeah! I 've been using // 5 amp Nicads with 10 ah NiMh on an Astro 112D charger for the past year or so and nothing has blown up yet!
otherDoc