I'm looking at just about any assembled li-ion battery pack for sale and they all have 2 connectors coming out (4 wires total) and I'm wondering why is that?
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5\$\begingroup\$ Extra two wires for a temperature sensor. \$\endgroup\$– Andy akaCommented Sep 25, 2019 at 11:55
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10\$\begingroup\$ I'm saying nothing of the sort and YOU are not supplying details of the specific pack you are investigating. \$\endgroup\$– Andy akaCommented Sep 25, 2019 at 12:03
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1\$\begingroup\$ @php_nub_qq Whether there is actually a temperature sensor in the pack may or may not be so; the 4 wires are standard and so are provided (and also make it seem as if the battery meets regulations). \$\endgroup\$– Peter SmithCommented Sep 25, 2019 at 12:04
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2\$\begingroup\$ Yeah, you should really ask the seller. If they can't answer an important question like that, then simply don't buy from them. \$\endgroup\$– DampmaskinCommented Sep 25, 2019 at 12:12
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3\$\begingroup\$ Looking at that eBay unit, it appears to me that the connector with the red/black wires is to be permanently attached to the ebike circuitry while the black cable with the barrel connector is for connecting the external charger. Presumably they are simply in parallel. Assuming you are looking exclusively at ebike packs, that may be a de-facto standard. \$\endgroup\$– DoxyLoverCommented Sep 25, 2019 at 13:02
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2 Answers
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Li-ion Ebike batteries often have a built-in BMS (Battery Management System) or PCM (Protection Circuit Module) that prevents over-charge, over-discharge, high temperature or excessive current draw, and may also have a balancer to keep all the cell voltages equal. As the normal discharge current may be 40A or more, Some have a separate connection that provides extra protection for low current charging.
Here is an example:-
28S LiFePO4 Li-ion smart bms pcm for 28s battery pack
Cheaper batteries sold on eBay etc. might have direct connections to the battery for discharge and a low current BMS for charging, or the charge port might be connected directly to the battery and is for convenience only. Without seeing inside the pack it's impossible to say what a particular battery has if the supplier can't tell you. Buyer beware!
answered Sep 25, 2019 at 15:20
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\$\begingroup\$ So basically I'm going to have a hard time using the load path for charging in regenerative braking, I will have to design it in a way that it feeds into the charger circuit, given the image in the answer. I'll probably get a cheaper-ish one from ebay for the sake of it and see what's going on inside anyway. \$\endgroup\$ Commented Sep 25, 2019 at 15:39
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1\$\begingroup\$ The discharge connector can also be used for fast charging, as it won't have the lower over-current protection of the charge port. Essentially it can be considered a direct connection to the battery under normal circumstances. You probably should monitor regen voltage and current separately, particularly when the battery is close to fully charged (since it cannot take any more charge without going over-voltage). This could occur if eg. the bike was run down-hill with a full charge. \$\endgroup\$ Commented Sep 25, 2019 at 15:57
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2 wires connect to the battery, and in general the extra 2 wires connect to a thermistor to allow temperature sensing of the battery. Although for more efficient wiring this could be done with a common ground giving a total of 3 wires, which is rarely seen.
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1\$\begingroup\$ It might seem efficient but it would make the thermistor signal bounce around with the voltage fluctuations at the battery end of the ground wire caused by changes in load/charge current. \$\endgroup\$– FinbarrCommented Sep 25, 2019 at 15:31
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\$\begingroup\$ Common connection is often used in Nicad/NiMH packs where the temperature sensor is monitored during charging only, and the charger can stop charging for a short time while measuring temperature (or doesn't care if the temperature reading isn't exact). \$\endgroup\$ Commented Sep 25, 2019 at 16:04