Hello,

I have a secondary (cheap chinese) battery hooked up in parallel to one of my ebikes for extended range. It is perhaps a year and a half old. Recently, the range of this bike has become drastically reduced. When I disconnected the batteries from one another, the voltage of the main battery that came with the bike was several volts lower (~46v) than the extended battery (~50v). The bike's display read 46v as well, so I initially assumed the secondary battery just became disconnected during a ride. I was able to ensure that all connections were fine, the batteries are connected via their outputs and are charged from one of either battery's charging port. I charged the main battery up to 50v, hooked them back together and continued to charge to 54v ... Now, when I disconnect the batteries from one another, the main battery reads 54.1v and the secondary battery reads 52.3v. How is this possible? I thought two batteries connected in parallel would equalize each other out, and with a 2v disparity, shouldn't something bad be happening?

On the other ebike, which is configured exactly the same, voltage always reads exactly the same on both batteries, regardless of where they are sitting in the charge cycle.

I am hoping someone can tell me if I perhaps have some bad cells, and based on the math how many? The battery is 48v (in marketing speak) and is composed of 18650's arranged in (I think?) 13s7p.

Here is the battery:
https://www.amazon.com/gp/product/B094ZCF4BL/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

 

how did you measure their voltage? you said you took them out of circuit. i guess then you measured voltage on each of them directly using DMM. of course measuring them just like that would be wrong. the correct way is to also measure battery voltage while battery is under load. if there is a discrepancy, your battery pack has defective cell that has high equivalent resistance. that one cell can render entire pack useless. it does look fine when the load is just DMM configured to measure voltage. but it fails to perform when current is drown from it.

 

Welcome to AAC!

Putting batteries in series or parallel always has problems.

Presumably you put the secondary batteries in parallel with the originals. Since they weren't matched and of equal capacity, how you charged them probably caused the short run time complaint.

This article might give you some useful information: BU-302: Series and Parallel Battery Configurations - Battery University

I have a 12V SLA that I rejuvenated using a battery maintainer. I put it in parallel with the battery from my truck that went bad after over 16 years hoping that the SLA would keep the maintainer trying to charge both; instead of declaring the truck battery is bad and turning off. The truck battery isn't taking any charge, but the SLA is.

I used that paralleling trick to recover a couple SLA's. I put them in parallel with a car battery that the maintainer "restored".

 

Yes, DMM with no load. I don't understand why this is wrong. Two batteries connected in parallel and under no load should have the same exact voltage, shouldn't they? I disconnect the secondary battery and test after the bike is has been sitting and powered off for a while. The primary is still connected to the bike but the bike is off, and the secondary one is completely disconnected. The other bike always has both batteries sitting at the exact same voltage when I check this way. This one used to as well, but no longer does. Overall range is certainly decreased. I just got back from a short ride and the voltages are the same again (not under load) but I will have to try going another five or ten miles to see if the voltages separate from one another after the bike sits for a bit.

Yesterday morning when I got the 50v secondary and 46v primary, the bike had been sitting with both batteries connected all night after a ride the previous day. I don't understand how I can get such different voltages when there has been no load for hours and hours. I was afraid to reconnect them until I manually equalized the voltages.

If the voltages differ after today's ride, then I would like to open it up and check for defective cells. I hope youtube can show me how to do this with the tools I have ...

 

Since they weren't matched and of equal capacity, how you charged them probably caused the short run time complaint.

I used that paralleling trick to recover a couple SLA's. I put them in parallel with a car battery that the maintainer "restored".

Maybe. I haven't had any problems with charging for the last year. I have always charged them extremely slowly with the charger that came with the ebike. To charge both batteries, it is probably about 11 hours or so.

What are SLA's?

 

I have always charged them extremely slowly with the charger that came with the ebike. To charge both batteries, it is probably about 11 hours or so.

Charging batteries in parallel or series is always a compromise. A bad battery in parallel with a good battery won't charge or could damage good batteries.

What are SLA's?

Sealed Lead Acid.