RCME - Parallel Universe in January 2017

[MattyB]

Posted by Bryan Anderson 1 on 15/04/2017 15:40:36:

...I would also want the batteries to be at similar terminal voltages otherwise the transfer of charge between batteries is not well limited. Even 3S 2200maH batteries that have open-circuit terminal voltages that differs by 300mV can sustain currents in excess of 30A for possibly up to a minute when connected in parallel. That is about a 14C charging rate.

In theory yes, but in practice that isn't the case because your maths doesn't take into account the voltage dip under load. When real world tests have been done (example, bottom left of page) you always find the equalisation currents are significantly lower than the theoretical values calculated, and the equalisation is pretty rapid (a few seconds before the current gets down to much lower levels). The effective charge current received by the lower voltage packs is also minimised if more than 2 packs are being connected in parallel.

Yes it's still plenty enough to fry a balance lead, but with most batteries now rated at 3-5C charge rates connecting them together in parallel should not cause any issues if the difference is less than ~0.2V/cell. Personally I am still going to stick with my 0.1V max total pack difference routine though, as it takes minimal extra time to execute and is proven with zero issues over hundreds of charge cycles.

Edited By MattyB on 21/04/2017 00:24:03

[Robert Hynes]

I liked this Matty. Particularly the link. Thanks for posting.

[Bryan Anderson 1]

Posted by MattyB on 21/04/2017 00:16:54:

In theory yes, but in practice that isn't the case because your maths doesn't take into account the voltage dip under load. When real world tests have been done (example, bottom left of page) you always find the equalisation currents are significantly lower than the theoretical values calculated, and the equalisation is pretty rapid (a few seconds before the current gets down to much lower levels). The effective charge current received by the lower voltage packs is also minimised if more than 2 packs are being connected in parallel.

Edited By MattyB on 21/04/2017 00:24:03

The voltage drop under load is caused by the internal resistance of the battery. The battery acts like a very large capacitor in series with that resistance. It takes added or subtracted charge - the integrated current - to change the charge state. The current, initially, is limited by the net resistance of the internal and external connections and the battery voltage difference.

How long do the currents flow? A 5000maH battery discharges at 83A constant current in about 3.5 minutes. To equalise the charge states when there is a 20% difference in charge state might take about 40 seconds - long enough to get things hot. Of course the current drops exponentially as the states equalise but that just prolongs the time it takes overall. This is easy to check - just connect and then disconnect after, say, 10 seconds and then re-check the voltages after letting the batteries settle.