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I run two 12 V LiPo starter batteries in my race bike. They only power a starter motor and the ignition in a total loss system. They are rated at around 10 Ah each and are different brands. They are connected thru' a marine switch so that I can switch briefly from battery 1 thru' "both" to battery 2. This enables me to start the motor on #1 and then switch over to #2 to run the coils thereby extending capacity without wiring them in parallel which is not recommended. Is this safe or could it damage the batteries due to brief voltage surges when switched thru' "both"?

winny
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Rick
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  • parallel not recommended? why? – Jasen Слава Україні Aug 23 '23 at 05:01
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    Depends how the switch is made. – winny Aug 23 '23 at 05:16
  • So in one position you could connect a full battery in parallel with a discharged one? No, that's not safe. A huge current could flow between them. – Finbarr Aug 23 '23 at 05:46
  • You either need to have them solidly connected in parallel, or never connected in parallel. Allowing them to get to different charge states, then switch briefly from battery 1 thru' "both" is a very bad idea. If you are going to use them separately, then they need isolation, either with diodes, or FETs. As you want to power a starter with them, they will need to be very beefy devices. If you are going to solidly parallel them (best way to use them both), then get them to the same voltage first, perhaps connect an auto bulb between them to limit current while they equalise. – Neil_UK Aug 23 '23 at 10:07

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Connecting two different LiPo batteries with different levels of charge (assuming they have the same voltage rating) is a very bad idea.

The battery can be modelled as shown here:

enter image description here

As you can see, the battery can be modelled with an ideal battery and internal resistor in series (r). The LiPo batteries have a very small internal resistance. For two batteries with different levels of charging, the voltage of the batteries is different. If you connect two of these batteries with different voltage levels, you will end up with two voltage sources that are connected via a very small resistor. This will create a very high current from the battery with higher voltage to the battery with lower voltage.

If this current exceeds the maximum charging current of the battery with a lower voltage or the maximum discharge current of the battery with a higher voltage, the battery will be damaged. To be able to connect these two batteries, you need to make sure there is a big enough resistor between them to limit the current within the maximum rating of the batteries. Such a resistor should have enough power to be able to dissipate the power going through it and it might get hot (depending on the switching time).

Saadat
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