Any battery experts in the house?
Today's little tinkering fun was arranging my two Soco batteries in parallel. I hoped that it would deplete the charge more slowly if there were two in parallel.
So. I have a spare Soco battery connector, so I wired the positive to the cut-off switch and the negative to the controller's negative terminal. No little wires — just the positive and negative leads.
With just that second battery connected without the data wires, the bike turns on and will drive the motor, but the battery reads as 0%.
With both batteries in parallel, the bike turns on, drives, and shows 100% on the main battery as usual.
It drives exactly as it did before. And after 10 to 15 minutes, the charge showed 90%. This tells me that it's depleting the main battery just as rapidly as before I added the second battery.
Is it not drawing power from battery 2 when battery 1 is connected? What have I done wrong? And more importantly, if I leave these connected in parallel, will bad things happen? Any suggestions would be helpful!
It will pretty much drive in the same way as before only that you may go the double distance.
Reading this, I think I'm going to hook my DIY battery up via the xt60 connection in the original battery, like you suggested. Seems like a better option.
you can't really calculate the available capacity based on the first 10% drop (there is always a very slight error margin for voltage displayed, without load vs discharge voltage for eg), you will need to ride it down further, maybe 50-30% a few times, to get an idea of your new capacity
I'll try that. If the battery indicator is inaccurate, then I guess I could look for other changes, like total distance travelled and if reaches top speed?
My main goal was to relieve the stress on a single battery when I'm in 3rd gear, delimited, and maxing out the throttle. I was hoping for a more quantifiable result, but I'll keep testing.
it isn't that the meter is inaccurate, more that the initial voltage change (the first few % drop in charge), is not entirely uniform - esp when dealing with 2 batteries together; this is not an issue restricted to the soco, it is a general issue with rechargeable batteries (some battery monitors work harder to give you more accuracy than others, but there are so many different factors, including the change in temp as the battery discharges)
the same can be said at the other end, the last 10-20%
Interesting that you chose to wire the positive end of the lead to the cutoff switch. Is that where the original cable terminates?
I would suggest testing the two batteries with a multimeter to establish the voltage of each battery. I wouldn't carry on using it because if the two batteries ARE connected in parallel, and only one battery is supplying the controller, I'd worry that you then risk a surge of voltage from one battery to the other that equalises them.
Using a mutimeter, if you can see that the voltage has dropped on both batteries, then you know that both batteries are supplying power. If not, then that implies one of two problems. Either:
- your BMS on the spare battery is not allowing current to flow, perhaps because of a significant difference in voltage between the two batteries, or perhaps, because the controller rejects the secondary battery's BMS, or perhaps for some other unknown reason.
- or you have not successfully established a parallel connection between the two batteries.
It is possible that you have used 10% of the two batteries after 15 mins, especially if you're in mode 3 the entire time. I know from using the single battery configuration that I can get down to about 85% or so in mode 2 after about 15 mins. If you're going uphill, or carrying a particularly large weight, I can believe that you may have used up 6Ah of battery in 15 mins. Seems a lot but not totally implausible.
if the batteries are parallel, it doesn't matter which is directly connected to the controller, they both are - only when in series can you have the issue you describe (one battery charging the other - lithium batteries (not cells) in series is always a very bad idea ofc) ✌️ ... that isn't to say there aren't problems which can arise tho, lithium batteries can be jumpy
def worth using a multimeter to check the batteries regularly
Wow. Thanks for that info and suggestions. I definitely want to get more data by checking the voltage of each.
About the positive lead being connected to the switch, the switch is already connected to the controller. This means that both batteries are supplying the controller. When the cutoff switch breaks the circuit, the batteries remain connected in parallel, but only the negative terminal is connected to the controller.
I can't do any more testing on this because winter has finally reached Vancouver. We're heading into a deep freeze for a week or so.
you can still test discharge rates, just keep the batteries in the bike and test the voltage very regularly (once a day) and compare it to what the bike says (keep in mind charge maintenance cycles to keep them happy) - also, incase it isn't obvious, you cannot get a V reading from just one battery, while both are connected in parallel, you will always get the sum of both, you need to disconnect one to read them
...ofc if your garage or wherever it is stored is also subject to Canadian winter temps then dont do this, lithium and sub zero is a bad combo
You're so right. It suddenly dropped below freezing here, so my bike is stored inside. When things warm up again, I'll be out there testing and measuring.
