Battery questions

Question

I have some questions regarding making a battery pack for a electric bike. I’m looking at the Samsung 50E 21700 5000mAh 9.8A Battery as the base.

1. The max discharge current is 9.8A, if I put two of them in parallel, will the max discharge current be 19.6A?
2. How do BMS work to charge the batteries safely?
3. What are some saftey precautions I should take to keep the pack safe?
4. If I have a pack of series of 4 of the batteries to get 14.4V at 5000mah, and I connect a pack of 4 parallel batteries to get 20000mah, can I connect them in series to get 14.4V 20000mah or will that damage the batteries because of the big voltage differences. And if not, how can I get a desired voltage of 48V and 25Ah? Any help is greatly appreciated. Thank you.

Answer 1

2. BMS is a balancing that depends greatly on how well matched the batteries are. If they are mismatched there will be problems to overcome due to heat dissipation limits when the weakest cell has reach full charge while the others must be in CV mode where the current is declining. If it is still in CC mode, you have a big heat problem to dump 4.2V*CC amps.

Since the weakest cell charges up the fastest, it also discharges the fastest making it even weaker long term from undercharged condition before the string reaches cut-off.

So there is an exponential runaway on the weakest link in a string especially when aggressive charge and discharge currents and CV and cutoff voltages are used to squeeze the max Wh out of a cell or pack. This leads to frustrated users with short life cycles.

You have a lot to learn about batteries before you can design them.

Answer 2

1. Yes

2. They protect against over charge, over discharge, over current and some of them balance the cells which is needed every once in a while.

3. Never charge them unattended, don't charge in cold weather (below 5C), don't charge in hot weather (above 40C), ensure the connections are well secured, ensure the BMS is correctly set for the voltage and current values, never use the pack without the BMS, and always triple check connections while building it.

4. No to get 14.4v at 20AH you need 16 cells, 4 in parallel then in series like this:

^{simulate this circuit – Schematic created using CircuitLab}

To get a 48V 25Ah battery you need 65 Samsung 50E 21700 5000mAh cells in this configuration:

^{simulate this circuit}

Basically you are connecting 5 cells in parallel to form a 3.7V 25Ah battery, then repeating that 13 times and connecting those batteries in series to get the finished battery.

May i ask what is the use case for such a battery? It is a 1.2kWh pack, not a small size.