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I apologise if this insults anyone's intelligence, but I've always believed the only stupid question is the one you don't ask, so here goes...

As a bit of background, I asked previously about powering a 60w heating element (10v 6A to be precise) and was advised the problem would be solved by using a bigger battery and this turned out to be the case. I bought a lithium polymer battery rated at 12v 10000mAh and that worked, and I heartily thank all of you who helped me with this.

My new issue is this - my (admittedly limited) understanding of battery terminology is that the Ah rating of a battery is how many amps the battery will supply for one hour, so for example a 5Ah battery will supply 5 amps for one hour, 2.5A for two hours and so forth.

I believed that, as the heater needed 6A (or 60w) to operate, if I bought a 12v 8000mAh battery, that would power it, but it didn't; the overload protection on the battery kicked in and prevented it from working.

So my question is this: am I right in thinking that the battery rating of 12v 8000mAh means that this battery should technically be able to supply 8A for one hour? If this is the case, I'm going to ask the vendor for a refund as the battery is not supplying the power advertised. If this is not the case, how do I calculate what size battery I need rather than just keep buying more expensive and (more importantly) bigger and heavier 10000mAh batteries?

As an aside, I am planning to test the batteries by hooking them to heating elements that have known power requirements so I can see exactly how much current they will actually provide before they cut out as opposed to what it says on the label.

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  • \$\begingroup\$ (1) Somewhat related thread, and this one too. (2) If you make an assumption that a 1Ah battery should be able to supply 1A, that wouldn't be a valid assumption. There are perfectly fine 1Ah batteries, that supply 0.001A. (finally) Please post the datasheet for your battery. \$\endgroup\$ – Nick Alexeev Dec 15 '16 at 23:07
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Theoretically, and 8000mAh (8Ah) battery should be able to deliver 8 Amps for one hour, but the actual usable capacity will depend on how much current you attempt to draw, relative to the Ah rating.

Lead-acid batteries are normally rated assuming a 20 hour discharge. If you attempt to discharge one in one hour, you may only get 50% of the rated capacity.

I assume that your battery is a Lithium type, since you mention a protection circuit - in that case, the maker has decided on a maximum safe discharge rate, and you attempted to exceed it.

You should study the datasheet for your battery to see how much current you are permitted to draw.

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  • \$\begingroup\$ Thanks for that. I think I understand better now. So if, for example, I had a 20Ah battery, it would more likely be designed to supply 1A for 20 hours rather than 20A all at once. I didn't get a data sheet with the LiPo battery. I'll contact the vendor and see if they have one. So the info I need to look for primarily when buying batteries is the maximum safe discharge rate rather than the capacity. I suppose with the batteries I have I can still test them myself to determine what the discharge rate is by upping the draw on them until they cut out. \$\endgroup\$ – Stephen W. Dec 15 '16 at 23:39
  • \$\begingroup\$ @StephenW. - "I suppose with the batteries I have I can still test them myself " Yup. And that's the best way, since you will have put some effort into it and the results will be deeply familiar to you. \$\endgroup\$ – WhatRoughBeast Dec 15 '16 at 23:50
  • \$\begingroup\$ @StephenW. There are high-discharge batteries available as well. Most batteries are specified in terms of "C rate" or "capacity rate." A 10Ah battery specified for 0.5C can draw up to 5A of current, and a 10Ah battery specified for 2C can supply up to 20A of current. This should be given on the datasheet for the pack, or at the very least available from the manufacturer. \$\endgroup\$ – Peter Dec 16 '16 at 0:40
  • \$\begingroup\$ That's very useful to know, thanks. I actually tested the batteries I have (and made a youtube video in the hopes of helping others with the same issue) and figured out the discharge rate for each type of battery I have (1.8Ah, 3Ah, 8Ah and 10Ah) so now I know how much power I can draw from them before the protection circuit kicks in. Here's the video, if anyone's interested... \$\endgroup\$ – Stephen W. Dec 18 '16 at 18:46
  • \$\begingroup\$ youtube.com/watch?v=7PLe7NZTUL8 \$\endgroup\$ – Stephen W. Dec 18 '16 at 18:46
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In addition to a battery's rated capacity the theoretical max discharge current the battery can deliver depends on the internal resistance of the battery. The internal resistance is based on a few factors such as electrode material and geometry. But the manufacturer usually also specifies a "safe" discharge rate that prevents damage to the battery if too much current is drawn (heat damage, chemical changes to the electrodes) so that it reaches it's specified lifetime. E.g. larger internal electrodes allow higher discharge current, but they would also lead to a higher self-discharge rate. So they would be good let's say for e-cigs or vaporizers which require a high current, but for devices drawing very little current over a long time these would not be ideal. In your case it seems there is also a protection circuit in play that limits the discharge current to a safe value for the battery.

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