Differences in charge when connecting batteries in parallel

[Refinished]

I have some 200ah 24v batteries that I'd like to put in parallel to increase capacity. How close in voltage do they need to be before I can connect them? Since the relationship between voltage and charge is so flat in LiFePO4 batteries, I guess I am concerned that a small difference in voltage will still result in a huge rush of current as they equalize. Is this a concern?

 

[Craig]

Short answer is I don't know. But seems to me if max output current is same as max input current it shouldn't matter how far apart they are. I'm interested in specs if you have them. Plus obviously connections need to be able to handle the load

 

[grizzzman]

I have some 200ah 24v batteries that I'd like to put in parallel to increase capacity. How close in voltage do they need to be before I can connect them? Since the relationship between voltage and charge is so flat in LiFePO4 batteries, I guess I am concerned that a small difference in voltage will still result in a huge rush of current as they equalize. Is this a concern?

I would suggest bringing each bank to a full charge then connect them.

 

[FilterGuy]

I would suggest bringing each bank to a full charge then connect them.

After you get them to full charge and as close as you reasonably can in voltage, get a 10 watt, 2 ohm resistor and put that between them till they equalize in voltage. That will limit the current while they equalize.


This drawing is for 12 volt packs but it all works exactly the same.

Even if there is 2 full volts difference between the two batteries, the current is limited to 1 Amp. As the batteries equalize, the current will drop.

Note: Depending on how out of balance the two packs are, and how large the packs are, it could take a while to equalize. Just plan on leaving them overnight and then see if they have equalized.

If you don't have a 2 ohm resistor, use something larger. A 10 ohm 2 watt could work if your voltages are starting within a volt or less of each other. However, with a larger resistor, the time to equalize will be longer.

 

[Craig]

After you get them to full charge and as close as you reasonably can in voltage, get a 10 watt, 2 ohm resistor and put that between them till they equalize in voltage. That will limit the current while they equalize.

View attachment 4980
This drawing is for 12 volt packs but it all works exactly the same.

Even if there is 2 full volts difference between the two batteries, the current is limited to 1 Amp. As the batteries equalize, the current will drop.

Note: Depending on how out of balance the two packs are, and how large the packs are, it could take a while to equalize. Just plan on leaving them overnight and then see if they have equalized.

If you don't have a 2 ohm resistor, use something larger. A 10 ohm 2 watt could work if your voltages are starting within a volt or less of each other. However, with a larger resistor, the time to equalize will be longer.

could you not theoretically go as high with the current as the lower of the Discharge or Charge C rate.

 

[FilterGuy]

could you not theoretically go as high with the current as the lower of the Discharge or Charge C rate.

Yes, you could. You just need a low ohm resistor with the wattage to handle it.

 

[newbostonconst]

Not sure how you could place a cell balancer between them when not charging but would be cool to use the new version the moves power between cells.

 

[SolarRat]

How close in voltage do they need to be before I can connect them?

I found out they don't need to be very close. I use 22 gauge jumpers...

https://www.amazon.com/gp/product/B06XDM8BY1/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

 

[FilterGuy]

I found out they don't need to be very close. I use 22 gauge jumpers...

https://www.amazon.com/gp/product/B06XDM8BY1/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Yup, I was just doing some quick math using typical internal resistance of LiFePO cells and it turns out that even at several volts difference you probably would not exceed the current of 12V 100Ahr batteries.

Having said that..... I would still let them equalize with a resister. (Don't ask me to justify that.....it just feels wrong to hook them directly if the voltage is very far apart and it is very easy to let them equalize first. )