Harleyflyer

My Feedback: (9)

I am planning on using a 12 volt battery (maybe a jump box) to charge up to 3 2600 lithium ion packs. The charger that I have can charge each of these packs up to a max charge rate of 3 amps while I am at the flying field (I wouldn't charge that fast but let's use the 3 amps for illustrative purposes).

I have been reading some articles off the internet and it says that a conservative charge rate is the amp hour rating of the 12 volt battery divided by 10. This particular article said you could take the amp hour rating and divide by other factors such as 6, 4 or MAYBE 3, as this article put it. My understanding from reading this was that the more amps that you pull out of a battery, the less efficient (the more heat it generates) it becomes and the more liklihood that you will not be able to realize the batteries maximum capacity for lets say a 12 volt battery. So, the way I understand this, that means that if I wanted to charge my 3 packs using the 3 individual ports charging at the maximum of 3 amps which is 9 amps in total.....I would need AT LEAST an 27 amp hour jump box or battery (27 divided by 3 = 9). And in my example here, this would be at the absolute maximum I should be charging on this 27 amp hour battery. Is this true? And if it is true, then if I were to say charge at only 2 amps max per port for a total of 6, then I guess I could get by with an 18 amp hour battery (18 divided by 3 = 6). If I have my battery facts wrong, hopefully someone can get me straightened out.

Sorry for the long post but wanted to give a detailed example.

I appreciate any responses in advance,
Keith

Red Scholefield

My Feedback: (9)

USING LEAD ACID BATTERIES AS CHARGING POWER SOURCE
I get lots of questions about how many charges can someone get out of their lead acid battery.
It comes down to the simple matter of how many watt hours are available from the lead acid battery. If we take a standard field box battery that many use for starting and fuel pumps, these are 12 volts, 7 Ah. 12 X 7 gives us 84 watt hours. Now to charge say a 3 S2100 LiPo pack it would take the nominal voltage 11.1 volts and multiply it by the capacity 2.1 AH and come up with 23.3 watt hours. But since charging is not 100% efficient nor are the chargers we use we take about 60 to 70% of the lead acid watt hours, in this case 55 watt hours and divide it by the watt hours to charge the 3S pack and come up with about 2.4 charges. Your mileage may vary but this gives you an approximation of how many charges you can get. Obviously if you don’t discharge the LiPo pack all the way you will get more charges. The number of chargers you get will also increase somewhat as the charge rate is reduced. Did I say it was simple?

Harleyflyer

My Feedback: (9)

Thanks Red. I see what you are are saying.

In my case, I was initially concerned with the fact that I did not want to try to pull out more amps from the battery with my charger than the battery would like to have pulled out at the same time. As long as I can safely and reliably pull out lets say a total of 6 amps from an 18 amp hour battery, I would be totally happy with that. It seems like I could do that right? Also, I realize I have a finite amount of charging that I can do off of it.

Thanks,
Keith

Red Scholefield

My Feedback: (9)

You can pull a lot of amps from a lead acid battery without damaging it. When you get too high it will tell you as it will not support the voltage you want. The harder you discharge it the lower the voltage delivered. A 7 Ah lead acid battery should be able to give you at least 20 amps for a short period.

Harleyflyer

My Feedback: (9)

Great...thanks Red! I think I am good to go now!

Keith