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I have a pair of these Li-ion batteries. I'd like to couple them so that I get double the current. However, I realize that this could be a Bad Idea™, as the charge could flow in a loop through both batteries.

What is the best way to prevent this?
Should I add diodes in series with each battery? (if so, what type and value?)
Should I put a resistor in series with one of the batteries?

Or is there a better way?

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The resistor solution has the main disadvantage that there can flow a (rather large) current from one battery to the other. This may reduce the life of the battery receiving the current (you're not properly "charging" it).
I would go for the diodes. If one battery's voltage is higher than the other this battery will be the only one supplying current, until both voltages are the same. From that moment both batteries will supply current.

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    \$\begingroup\$ With diodes though you are effectively reducing the voltage out of your battery by 0.4v ish \$\endgroup\$ Commented May 2, 2011 at 16:37
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Two batteries in parallel will work fine if the following are true:

  • the batteries are of identical types
  • the number of cells in each string is low (1 is best, 2-4 OK, I wouldn't do this for >4)
  • the cells are matched (same age, charged + discharged together)
  • the cells are in thermally similar environments -- if they're next to each other but one is next to a heat sink and the other is on top of a hot component, you'll reduce life.
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If the load power isn't too low, you could connect the batteries in series then use a converter (i.e. a buck) to convert the power to lower voltage / higher current. You'll need to implement some sort of undervoltage protection to shut the converter down if the input voltage gets too low. You'll lose some power in control as well as conversion.

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Connecting 2 batteries of similar charge in parallel should not be an issue, your charger may struggle though and you may have to charge them seperately.

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The original post hinted at a problem with parallel charging I experienced. On of the three 12V 100Ah batteries developed a bad cell(s) and essentially dropped to 4V. This caused the other batteries to die trying to equalize the pack. In the end, I lost three pretty expensive batteries.

My current solution is to use a smart charger that has the capability of charging many batteries using separate leads to each battery. That avoids the problem of parallel charging by not parallel charging.

If you are going to parallel charge two or more batteries, you need to install a device that prevents energy from backflowing from one battery to a weaker battery. This is accomplished with a resistor (sized for your project) or a MOSFET (also sized to your project) which is installed in line with the connections between the batteries.

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  • \$\begingroup\$ yes Series and shunt protection is needed for series and shunt failure modes. (OC,SC) \$\endgroup\$ Commented Sep 11, 2016 at 17:31

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