Voltage delta?

[Lincs Robert]

Hi,

My newest Chinese stack is ready, 15 x 230Ah cells & sitting at an overall, cell balanced, voltage of 49 volts.

Question is, if I want to bring them into service in parallel with another stack, what’s the maximum voltage difference I should consider making the connection at? I’m assuming that ANY difference will cause a momentary stress to each bank? I could link with a low value resistance & let things equalise a while and them remove when there is zero difference?

Comments welcome.

Thanks- Rob

[nowty]

Just looking back when I added a 48V parallel string. I first tried at 3V difference and that was over 100A, maybe a lot over so I re-tried at 0.75V difference using a fuse to make the connection just in case. That was a more comfortable 50A or so.

I would say 1V or under (full stack voltage) would be ok.

Its always scary though.

[openspaceman]

Wouldn't it be a little less exciting to wire a big resister (like the element from an electric fire) between them, let them equalise and then join the busbars?

[AlBargey]

Can you avoid the playing with plasma by just charging both packs fully before connecting?

[HML]

I don't think it really matters much. The high current (which will be limited by the cable/connector resistance) will only last a very short time.

Andy from the off grid garage has done a video on this.

If your really worried use a 0.1 ohm resistor which will keep the current below 100A even if there's a 10V difference.

I didn't bother when I paralleled my 4 banks, but they were charged to fairly close voltages.

[Joeboy]

I seem to remember I charged my new pack so far (say 70%) then did a drawdown on the existing home pack to match as close as possible.

Even being a measured couple of hundredths of a Volt between them I still got a nice fat spark on final connection.

No harm done as far as I could see. I guess with hindsight I'd try and match packs at the lower end of the scale but that'll be kind of dictated by the soc of the new to be connected pack once its balanced?

[Lincs Robert]

Wouldn't it be a little less exciting to wire a big resister (like the element from an electric fire) between them, let them equalise and then join the busbars?

I did make that comment in my OP

[Lincs Robert]

I seem to remember I charged my new pack so far (say 70%) then did a drawdown on the existing home pack to match as close as possible.

Even being a measured couple of hundredths of a Volt between them I still got a nice fat spark on final connection.

No harm done as far as I could see. I guess with hindsight I'd try and match packs at the lower end of the scale but that'll be kind of dictated by the soc of the new to be connected pack once its balanced?

I think that’s the way I’ll do it tomorrow. A combination of my “low resistance” and using the electric oven to drawdown ……..

[Joeboy]

I seem to remember I charged my new pack so far (say 70%) then did a drawdown on the existing home pack to match as close as possible.

Even being a measured couple of hundredths of a Volt between them I still got a nice fat spark on final connection.

No harm done as far as I could see. I guess with hindsight I'd try and match packs at the lower end of the scale but that'll be kind of dictated by the soc of the new to be connected pack once its balanced?

I think that’s the way I’ll do it tomorrow. A combination of my “low resistance” and using the electric oven to drawdown ……..

Remember to give the homestack time to settle to an accurate reading once you remove the oven load. A minute or two should do it.

Congratulations BTW!

[openspaceman]

Wouldn't it be a little less exciting to wire a big resister (like the element from an electric fire) between them, let them equalise and then join the busbars?

I did make that comment in my OP

Yes but I was thinking a higher resistance for a longer time and asking more than suggesting.