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I have purchased an auxiliary 18 Ah lithium battery to supplement the stock 13 Ah battery. For convenience, I would prefer to configure them in parallel for operation. The resulting slower drain should extend battery life. But there will be many situations where the batteries will be separated and become dissimilar in charge. I want to do something to stop or slow excessive reverse current flow that could damage my batteries.

Are Schottky diodes right for the task and can anyone draw me a schematic or share any relevant advice?

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  • \$\begingroup\$ this spplication is on an ebike, shouldn't matter \$\endgroup\$
    – Bruce Mcintosh
    May 10, 2020 at 14:13
  • \$\begingroup\$ The concern is that high current from the more powerful cell will damage or burn the less powerful one. Until they have the same voltage, the smaller one should be disconnected. So far I have had no such problem also and I use them without a relay too. But since its a 13A/h, it's better not to risk. allaboutcircuits.com/textbook/direct-current/chpt-11/… \$\endgroup\$
    – CFCBazar
    May 10, 2020 at 14:53
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    \$\begingroup\$ @CFCB, it's 13 Ah, not 13 A/h. \$\endgroup\$
    – Transistor
    May 10, 2020 at 15:02
  • \$\begingroup\$ actually @BruceMcintosh, it's almost always more useful to provide an application with your question. This question and my answer is (ironically) a pretty effective illustration of that fact \$\endgroup\$
    – Ocanath
    May 11, 2020 at 23:38

2 Answers 2

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You can connect two batteries like this ...

schematic

simulate this circuit – Schematic created using CircuitLab

... and the higher voltage one will power the load, until the voltages equalise and then they will both share the load current.

Neither battery will be able to damage the other in any way.

The batteries should be the same nominal voltage, otherwise you won't gain the benefit of both their capacities. For instance if you have a 2S and a 3S lithium, your load voltage would have to drop to a damagingly low voltage for the 3S before the 2S made much contribution at all.

Of course, as Chris pointed out in comments, the batteries now can't accept regenerative braking from the controller. This still won't damage the batteries, but it might upset the operation of the e-bike.

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    \$\begingroup\$ Not necessarily, the controller may assume it can dump energy back into the battery. \$\endgroup\$
    – Chris Stratton
    May 10, 2020 at 15:57
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An e-bike represents a seriously high power load. Schottky diodes do have a characteristically low forward voltage drop, but considering the peak power conditions of an e-bike motor, most schottky diodes would fail if used in an arrangement that would otherwise allow you to share unequal capacity batteries in parallel. There are large diodes that can sink serious amounts of power, but you're still talking about potentially 100's of watts wasted in a battery powered device.

A more practical solution would be to use an ideal diode controller, with some high power n-channel MOSFETs.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ note that this circuit is essentially the same in form and function as @Neil_UK's answer, especially if you consider the schottky diodes in that circuit 'ideal', and will therefore suffer the same problems wrt. regenerative braking. It is a more practical implementation of the same concept, so i think it's beneficial to show both \$\endgroup\$
    – Ocanath
    May 11, 2020 at 23:34

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