[Frank]

The batteries in my motorcycle sat from ~early December to this week. I pretty much left them alone but ran the DC/DC/ converter for a couple of AH through the winter to keep the 12 volt battery charged up. It's been in the forties the last couple of days so I went for a very short ride and decided to recharge. Per the Cycle Analyst I was only down 6-7 AH. So far I've replaced all that plus another 10 AH and the voltage levels are still 3.4-3.5 or so. I suspect I can put another 3-4 AH in before individual cell voltages start taking off.

The extra 10-14 AH has surprised me a bit - I'm wondering what others experience is in this situation. Is there a self-discharge phenomenon that's exaggerated by cooler weather?

TIA

 

[EVfun]

Not that I've seen. I suspect your cycle analyst simply fails to correctly capture the current at very low levels. I'm not familiar with the cycle analyst but I can give an example based on the a vehicle with a shunt.

A lot of EVs have a 500 amp shunt in them to measure current. If the actual draw pulled the pack down 6 amp hours in 6 months the current was about 1.4 milliamps. The 500 amp shunt is nothing but a precision low value resistor so that the voltage across it would be 50 millivolts at 500 amps. That is a 0.0001 ohm resistor. If only 1.4 milliamps are flowing across it the voltage drop would be only 0.14 microvolts, that is way under one millionth of a volt. I wouldn't expect a device designed to handle over one hundred thousand times that voltage (at least 50 millivolts) to accurately measure that.

 

[Frank]

That's the shunt I'm using but the 6-7 AH is after a short ride too, so it's possible the CA is misrepresenting things somehow. I use Celllogs to monitor voltage and leave them connected (but off.) I need to investigate how much current they draw (if any) in this mode. If anyone knows it'd be great if you could sound off.

I've always thought they drew power equally from all cells to run the device itself but I'm not sure about that. Any knowledgeable opinions on this one?

tia

 

[EVfun]

6 months and down an extra 14 amp hours could be accounted for with a continuous 3 milliamp load. Something like that is almost certain to be missed by anything measuring with a 500 amp shunt.

I'm not familiar with the Celllogs. You really want to avoid any non-equal loads between the cells unless you have shunting battery regulators (even then, they are not a good idea.) Oh, if you do have cell level regs that is almost certainly where the 10-14 amp hours went.

 

[Frank]

I don't have cell-level regs but a celllog input hooked to every cell. Still investigating to confirm power draw. Total draw was 6-7 AH after sitting for 3-4 months, running the DC/DC a few times and taking a 3 mile ride. I'm thinking internal resistance is playing a part in this also as lower temps definitely increase sag.

 

[MN Driver]

Cell logs draw their current from the first few cells from what I've read from the endless-sphere e-bike guys and I think here as well. Not sure what there is for standby power but I'd check the voltages of you cells toward the end of charge to see if they are near eachother and you don't have any going nuts with overcharging. You probably need to rebalance, I'd definitely check the balance of your pack.

When you said 3.4-3.5, is this measuring each individual cell, if so things might not be so bad but you'll want to check the cells at the end of charge when things are around 3.6 volts due to packs generally hanging in the mid-high 3.4 to low 3.5 for awhile while still picking up decent current.