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I bought some batteries for a project with the recommended voltage but too weak Ah.

The batteries are to be connected in parallel. I can't currently afford to upgrade both batteries, but I was curious if I can try upgrading one of them and connecting it in parallel to one of the ones I have now? My robot is going to weigh less than the one on which it is based, so perhaps just upgrading one is sufficient.

Or is that unsafe?

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You aren't being specific enough. Some chemistries would possibly allow this but others would not. If the batteries have the same nominal voltage than theoretically they should work fine in parallel with no issues. The problem is when you start discharging them.

See the graph below:

enter image description here
(source: mpoweruk.com)

Batteries with different capacities would discharge at different rates, so if you had two batteries in parallel of say 1000mAh and 500mAh, there would be a heavy imbalance in the rates of discharge, causing them to be at different state-of-charge percentages. If one were at 80% and another at 40% then there would be a huge issue because even though the difference in the voltages could be as small as .1v it would be enough to cause a short and cause heavy damage to your batteries. This is true for all batteries, though some like the NiMH batteries are more tolerant and could potentially survive if the currents aren't that far off. But definitely NEVER try this with a LiPo unless you are a fan of melting your hands off.

To quote SparkFun:

Be careful when connecting batteries in parallel! All the cells should have the same nominal voltage and same charge level. If there are any voltage differences, a short circuit could occur causing overheating and possibly fire.

TL;DR Hella unsafe, use at your own risk. I would not recommend.

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  • $\begingroup$ They are gel batteries; deep cycle $\endgroup$
    – Hack-R
    Commented Jan 31, 2017 at 5:25
  • $\begingroup$ Gel batteries have some of the steepest curves out there. See here. Not only would that have some of the largest differences in voltages and strongest shorts, but if one of your batteries are quite old the shorts could cause a sudden, powerful explosion. It won't be as painful as a LiPo melting you, but I certainly would not recommend doing this with gel batteries because of how much easier it is to short them when running different mAh batteries in parallel. $\endgroup$
    – Bakna
    Commented Jan 31, 2017 at 5:33
  • $\begingroup$ Thanks. For what it's worth they are all new, all have the same theoretical voltage and are all gel. But I guess I won't try it. $\endgroup$
    – Hack-R
    Commented Jan 31, 2017 at 5:35
  • $\begingroup$ Even new batteries can have major issues when they short (spewing acid everywhere). Stay safe and make on! $\endgroup$
    – Bakna
    Commented Jan 31, 2017 at 5:36
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I think you should always use a resistor to limit the current when directly connecting batteries in parallel. Since they will never start at the same voltage the current will start flowing from the higher voltage into the lower one (like when you jump-start a car battery) this could melt your cable connections if you aren't using a resistor until they stabilize at the same voltage.

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