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I have two 12v batteries, 7ah and 20ah. Can they be connected in parallel without damaging the 7ah battery? They will be charged separately.

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  • \$\begingroup\$ No, if you are putting 2 batteries of the same voltage in parallel, they must be the same capacity \$\endgroup\$ Commented Jul 21, 2017 at 20:11
  • \$\begingroup\$ It depends on the specs of each , their condition , state of charge and the load. So Yes and No. Batteries already have this property but it is possible certain combos of ESR*C with high stress loads may overstress the smaller cell \$\endgroup\$ Commented Jul 21, 2017 at 20:11
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    \$\begingroup\$ Probably if you can control the current while the voltages equalize. As an example, if you connect a fully charged 20Ah battery to a discharged 7Ah battery, you may get very large charge current. So don't do that! But if you only connect them when voltage is already equal, it will PROBABLY be OK. \$\endgroup\$
    – user57037
    Commented Jul 21, 2017 at 20:26
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    \$\begingroup\$ You didn't mention which chemistry(ies) they are. It really, really matters. \$\endgroup\$ Commented Jul 21, 2017 at 22:39
  • \$\begingroup\$ Re, chemistry, good point. I was assuming lead acid. \$\endgroup\$
    – user57037
    Commented Jul 22, 2017 at 3:51

3 Answers 3

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If you simply parallel connect the two batteries for discharging, then the combined Ah capacities do not add. As current is drawn from the paralleled batteries, the voltage of the 7Ah battery will typically drop more quickly than the 20 Ah battery. This will typically cause the higher voltage 20 Ah battery to begin charging the 7 Ah battery. Whichever way the current draw occurs, the action of one battery charging another battery wastes energy in the form of heat.

Since part of the energy of one battery is being used to charge the other battery, the combined capacity will likely be more than 7 Ah but less than 27 Ah. The exact answer is not known without an extensive analysis of the load characteristics and the specifications of the batteries.

By adding some electronics to the system, it may be possible to realize an overall higher Ah capacity compared to simply paralleling the batteries.

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    \$\begingroup\$ You probably assume that both paralleled batteries give equal current (because you wrote that voltage of one battery would drop faster than of the other one). They are in parallel so voltages are equal. But they have different internal resistances which will cause that each battery will supply different current. One battery cannot supply current and be charged at the same time. \$\endgroup\$ Commented Jul 22, 2017 at 12:42
  • \$\begingroup\$ @chupacabras I did not assume that nor did I state that one would supply current and be charged at the same time. \$\endgroup\$
    – Glenn W9IQ
    Commented Jul 22, 2017 at 12:47
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    \$\begingroup\$ Yes,you did. You wrote that voltage of one battery will drop faster. Which is not true since they are in parallel. And you wrote that one battery will charge the second battery which is not true either. Both batteries will supply current, none of the will act as a load. Just each battery will supply different current. \$\endgroup\$ Commented Jul 22, 2017 at 12:59
  • \$\begingroup\$ Have you tried this? Combining 12V batteries of different capacities in parallel? \$\endgroup\$
    – user57037
    Commented Jul 22, 2017 at 22:17
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    \$\begingroup\$ @Chupacabras, my question was directed to Glenn. It is my belief that batteries of same chemistry and design placed in parallel will share current more or less proportionally to their capacity throughout the discharge cycle. I do not see how it could be otherwise. In other words, I agree with you. \$\endgroup\$
    – user57037
    Commented Jul 27, 2017 at 3:55
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Yes they can be connected in parallel. Different Ah of batteries means they have different ampere hours, thus when used alone a 7Ah battery will discharge sooner than a 20Ah battery when connected to the same load.

When both are connected in parallel they become one 27Ah battery of 12V.

They can be charged from the same source of ~14V simultaneously. If voltages were different then the higher voltage battery would act as a source while the lower voltage battery would act as a load which is not recommended.

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    \$\begingroup\$ "Ah" (not AH or ah). "They caneben be charged from same source of 14 volt approx simultaniously." if they are lead-acid type. \$\endgroup\$
    – Transistor
    Commented Jul 21, 2017 at 21:25
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    \$\begingroup\$ If you decide to connect these two 12 volt batteries in parallel it is very important that they are at the same level of charge when you connect them. I.E. it is easiest to fully charge them both and then connect them in parallel. If one is empty and the other charged or paartially charged and you connect them in parallel, the charged battery will dump a large current into the empty battery - not good. \$\endgroup\$
    – Filek
    Commented Jul 24, 2017 at 0:48
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As a general rule I never put batteries (of same nominal voltage of course) in parallel: they might have different State of Charge (SoC), different internal resistance, etc. and the situation will turn out into a battery charging the other, rather than having the current all going where it is required to be (... your load).

To replace batteries on the spot and to protect against reverse polarity, I always use a diode in series, possibly Schottky to reduce voltage drop and heating.

schematic

simulate this circuit – Schematic created using CircuitLab

D1 and D2 may be different components, or the two legs of a common-cathode 2-diode component (see below).

  • With a normal PN diode and 10A flowing, forward voltage drop of 0.9-1.0 V is common, with 10W dissipated across the diode (!), and 1V less on your load.
  • Schottky diodes have a much lower voltage drop (about 50%): 1 leg of a common-cathode 2-diode MBR2045 at 10 A has a 0.58V voltage drop; with the 2 legs in parallel (5A each) and some self-heating that brings temperature around 50-60°C, 0.45V can be achieved => 4.5W. Vishay VS-48CTQ060S-M3 can reach 0.37-0.38V, thus 3.8W!

A connector suitable for current rating is always a good idea

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