12v 35ah battery question?? how to convert AH to mah?

[emederos]

I just bought a 12v 35AH pb battery for my charging box for the field and need to know how to get a general idea on how many 6s packs i can charge (estimate) with a full charge on my 12v battery. I am currently charging 3s packs like nothing and it barely tickles this battery but i know when i connect my 6s batteries it should draw a lot more juice from the battery being 22v batteries. My general question is how many 5000mah packs 6s would i be able to charge on a fully charged 35 AMP HOUR fully charged 12v battery? how can i calculate how many milliamps i can get out of this 12v battery and will the voltage on the 6s batteries kill the 12v battery being a lot less voltage?

[bstanley72]

The 35 ah battery would be 35,000 mAh. You will not want to discharge it all the way down, so 70-80% give or take. So let's say you have 25,000 mAh to work with. Since 6s batteries are roughly twice the voltage as your source it will take 2 mAhs out of the PB battery for each mAh put into the lipos. So you have 12,500 mAhs to work with. Assuming you discharge the 6s batteries to 20% you'll be able to charge one up three times off the PB battery. This allows a little extra for inefficiency in the system.

[emederos]

Thanks for the reply so i will be charging 6s 5000mah packs i should be getting about how many charges assuming they are 20% of capacity? With 35,000 mah thats roughly 5 or 6 leaving power for the charger etc...... how about the charge rate would that matter if i charge at 1c as opposed to 3c?

[Slyster]

Well.. with dual 110Ah deep cycles (24v) and I can get around maybe 14-16 charges (6s5000)...

With one 12V 35Ah.. you need to half the Ah (to double the voltage to match the 24v for 6s).. so with 35Ah.. I would say 2 charges.. MAYBE 3 one in a while if you push the deep cycle (but damaging the deep and anyways the voltage may drop off so fast the charger would stop). That's about it.

[emederos]

And on charging 3s packs 11.1v about how many charges am i looking at? Also does it matter at what charge rate i charge i.e.if at 1 c versus 3c will it drain faster or does it just go by capacity in mahs?

[Slyster]

3C will get fewer charges than 1C. I forget the reason.. there's some electrical law.. never understood why. What sized 3s packs?

[emederos]

2200 3s packs, my 6s packs are going to be 5000mah

[Slyster]

Essentially 2 2s2200's will be close enough to a 6s5000 that I'd say 4 maybe 5 charges?

[bstanley72]

You have 25,000 mAh to work with. So you can just do the math. 3s batteries will draw mAhs at a 1:1 ratio, so a 3s 2200 discharged to 20% will require 1,760 mAhs to charge it up. 25,000/1760 = 14 charges. 6s batteries will draw mAhs at a 2:1 ratio, so you have 12,500 mAhs to work with. A 6s 5000 mAh battery will require 4000 mAhs to charge it up. 12,500/4000 = 3 charges.

[emederos]

Very well explained got the idea now ....cheers

[RedOctobyr]

At some point, it makes more sense to actually start thinking of all this in watt-hours, which is really what is being described, in a slightly roundabout way.

Talking only in amp-hours, then doubling or dividing by two, works in this case, but is somewhat confusing, and becomes more so if you were talking about 4S packs, or something not a multiple of 12V.

A 12V, 35Ah battery has a stored energy of 12V * 35Ah = 420 Watt-hours, or 420Wh. It can supply 420 watts, for 1 hour. Or 840 watts for a half-hour, etc. But watt-hours are important because if this was a 2V, 35Ah battery, your total stored energy would of course be much smaller, and much less useful, only 70Wh.

If you didn't want to drain more than 70% of the battery (I've heard deep-cycles shouldn't really be drained below 50%, ideally), you'd have 294Wh available, of your 420Wh.

To be conservative, for LiPo calculations, let's use the fully-charged voltage.

A 3S (12.6V) 2200mAh stores 12.6V * 2.2Ah = 27.7Wh. If you drained it by 80%, you'd need to put back around 22Wh.

A 6S (25.2V) 5000mAh stores 25.2V * 5Ah = 126Wh. Drained 80%, you'd need to put back around 101Wh.

There are efficiency losses in the charger, maybe 10-15%. So maybe you effectively take another 10-15% off what you're considering the capacity of the lead-acid to be.

And you won't actually get the claimed output of the deep-cycle, because of Peukert's law, as Slyster alluded to. The faster you drain it, the less of the stored energy can actually be put to use. Maybe think of it like running. Run fast, and maybe you can run 2 miles. Slow down to a jog, and perhaps you can go 3 miles, or whatever.

So this is only going to provide guidelines. But at least we can now express all of the batteries in common units, rather than simply multiplying or dividing by 2. Which can help compare any deep-cycle or LiPo scenario you want to figure out.

[emederos]

True and makes complete sense. I was looking into voltage charts to have an idea on what capacity my 12v 36ah battery actually has i.e. at 12.7 volts under load its 100% and not under load should be over 13v , can you explain resting voltage vs load voltage and how long after a load can it be tested to get actual resting voltage?

[Slyster]

After a load it takes a good 2 full minutes for the voltage to settle down.

Only use the voltage AFTER that rest to determine how much capacity is left!

I made this chart from a few sources and have it printed out and stuck on my back window of my truck.

[Slyster]

Originally Posted by RedOctobyr

And you won't actually get the claimed output of the deep-cycle, because of Peukert's law, as Slyster alluded to.

True.. but I never understood this.. why? Where does the energy go? It must go somewhere.. do the deeps lose energy through heat loss (ie do they get warm under high load?)

[RedOctobyr]

"Under load" sounds like quite a generalization, to me, if using that voltage to estimate charge.

What size load? 1C, or some other "standard" that takes into account the size of the battery?

Using a LiPo as an example, a given load will cause a smaller voltage drop for a big LiPo vs small, or got one with a higher C rating vs a lower C rating.

I'd be reluctant to put much faith in reading the voltage when under a load that will vary, such as when charging. Charge a small pack at 1C, then a big pack at 3C, and you will measure different voltages in each case.

Sent using Tapatalk.

[emederos]

The PB battery charges up to over 13 volts does this voltage automatically drop while not in use to 12.6 or is the 12.6 100% capacity only under load?

[RedOctobyr]

I believe they drop off that way naturally after disconnecting them from the charger.

Sent using Tapatalk.

[emederos]

Originally Posted by Slyster

After a load it takes a good 2 full minutes for the voltage to settle down.

Only use the voltage AFTER that rest to determine how much capacity is left!

I made this chart from a few sources and have it printed out and stuck on my back window of my truck.

Nice chart going to reference this from now on cheers

[3D Dave]

Nice chart Pete.

I'm assuming I can just extrapolate those figures down to 2s and 1s (and expand the chart on my own) so when I am checking the voltage of my smaller lipos after a flight, I can verify the percentage (since I'm so used to remembering the 20% left rule verses 7.72v on a 2s pack or 3.86 on a 1s pack).

I'm so used to using my Hyperion meter (since it's easier to see the digits and remember the percentage). I also have the 8M celllog but the display is so small & hard to read I tend to use the Hyperion.

I'm going to make a similar color coded chart for my lipos.

[RedOctobyr]

Originally Posted by 3D Dave

Nice chart Pete.

I'm assuming I can just extrapolate those figures down to 2s and 1s (and expand the chart on my own) so when I am checking the voltage of my smaller lipos after a flight, I can verify the percentage (since I'm so used to remembering the 20% left rule verses 7.72v on a 2s pack or 3.86 on a 1s pack).

No, Pete's chart is for lead-acid batteries only, and has no bearing on LiPos.

My mental guideline for LiPos is 3.85V is 50%, and 3.7V is approaching 20% left. It's probably not quite as low as 20%, but that doesn't bother me, I don't run them as low as 3.7V anyhow.