Dual Battery Connection Adapter

[saneagle]

We are miscommunicating.

I was talking about my weak cell group which i am convinced is caused by a single dodgy cell inside a group of six that are in parallel. My point was about how that parallel connected group must be flowing current into the weak cell.

I know how series charging and balancing works.

That would only happen if the one cell had an internal short, but then it would go all the way to zero.

You guys come up with all these clever theories. Why don't you try actually opening a battery and measuring things, then you find out stuff? I have never found even 0.01v difference between cells in a parallel group while charging. They're all held in parallel by plates of nickel sheet, so they can't be at different voltages to charge each other. it doesn't matter which one you measure, they're all the same.

 

[Ghost1951]

I have never found even 0.01v difference between cells in a parallel group while charging. They're all held in parallel by plates of nickel sheet, so they can't be at different voltages to charge each other. it doesn't matter which one you measure, they're all the same.

Of course they stay the same voltage. They are connected together.

That doesn't stop my dodgy group being dragged down in voltage by comparison to the ones it is connected to in series. The whole group is affected.

This morning, after being left unused for three months while I have been riding the Argos folder, the overall battery voltage was 40.2v while my bad group was 3.82v. It isn't that the weak group is powering the BMS while it is sitting unused because it falls quite rapidly when fully charged and put to use. Even when put back in balance, that group drops about 0.2v after five miles by comparison with the others.

Generally, the BMS puts it back to near balance and now and then I charge that group on its own when it gets too far out of line. I have two fused wires leading to insulated contacts on the outside of the case which I can use to monitor and charge the group when needed.

 

[eas2lv]

Part of the confusion here is not distinguishing between the emf of the battery and voltage in the circuit. Batteries connected in parallel will have same voltage in the circuit but can have different emf under load because of the drop across the internal resistance. When load is removed, no current is flowing, and emf could be different and will try to equalize.
That said, if you start with batteries having same emf, account for the emf vs capacity relation and note that capacity change is equal to integral of the current, they have to be in dynamic equilibrium at all times. That does not allow an emf mismatch to develop. I think this @saneagle 's point.
But something catastrophic can happen where one battery loses all capacity to generate an emf and acts like a passive resistor. Use of a diode protected Y will guard against such occurrences.

 

[Nealh]

Sorry to be a party pooper, but I must ask...

Why on earth would you want two batteries on your bike???

I had two batteries for longer rides , 160km from one battery wasn't pssible for me.

 

[Nealh]

I had two bike set ups with parallel batteries and at the end of each journey both batteries on each bike was in much the same ball park voltage wise.

My Boardman had a PF 14.5ah cases battery and a softpack of PF 's 10s 8p a simple Y lead connected them but always made sure charge voltage was within 0.02v before connecting up.

The Swizzbee ended up with a bespoke PF pack I made and fitted in to the frame case, 23.2ah and a HE2 pack of 17.5ah sat in the rear top bag . Like wise as with the above final SOC were much the same.

 

[Jodel]

My knowledge of the electronic / electrical side of things is rather less than comprehensive, so I like to keep it as simple as I can.

I've never really had a range problem on the tandem as we rarely go more than 30 miles these days, but having access to a second battery to swap over is quite comforting if I want to put the power up a bit to combat headwinds. I can fully understand the comments about voltage sag, but again that's never been a rate limiting factor for me (yet).

One of the plus points of a tandem is the size of the frame - mounting a second battery isn't much of an issue.