Charge rates

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Re: Charge rates

Postby RCModelReviews » Mon Aug 22, 2011 8:40 am

Yes, the charge current will drop off as the battery becomes nearly charged.

Lithium batteries are charged using a process called constant-current constant voltage

What this means is that the charger will apply only enough voltage to produce the maximum charge current you have selected until such time as that voltage reaches 4.2V per cell (that's 8.4V for a 2S pack, 12.6V for a 3S, etc). It will not increase the voltage beyond this point so, as the battery continues to charge, the current will automatically begin to reduce -- until, when it reaches some pre-determined minimum current (normally less than 100mA), the charger will say "full".

As for the the number of amps it'll suck out of your car battery...

Well think in watts again.

If you're charging at the full 50W that your charger can handle then it will be drawing 50W/13.6V (assuming you have a fully-charged car battery that is putting out 13.6V) which is about 3.7A. That's even if you're charging a 6S pack at 2A -- which is also about 50W.

In order to allow you to charge batteries that are a higher voltage than your car battery, the charger includes a voltage boost circuit that converts the 12V of your car battery into as much as 30 or more volts (depending on the charger). However, since it's all about watts, 1A of current at 30V is 30W and 30W at 12V is 2.5A so the charger will draw more current than it appears to put into the battery.

So, to figure out the maximum current your charger will suck from your car, find out how many watts it is and divide by 12. That assumes you're running the charger at full wattage and that your car batter isn't fully charged.

A 50W charger will suck a maximum of around 4A
A 300W charger would suck a max of around 25A

and you can calculate the rest for yourself.
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Re: Charge rates

Postby ergocentric » Mon Aug 22, 2011 2:43 pm

jpreou wrote: except at the end when it slows right down.


it doesn't "slow down" so much as it is transitioning to constant voltage charge mode from constant current

the current will have to go down to keep that constant voltage at the end of the charge
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Re: Charge rates

Postby jpreou » Mon Aug 22, 2011 10:12 pm

Yep, aware that 'slow down' is not technically correct but it is a good approximation, sort of.
Thanks for all the info guys.
Bruce; hope to catch up with you again at the weekend, weather permitting. Might even have my 'new' 'part-custom-part-plan-build' ready to receive the radio gear (which you have!).
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Re: Charge rates

Postby RCModelReviews » Mon Aug 22, 2011 10:16 pm

Let's hope the weather is good. After the last week I need to do some flying. Wife is in hospital for more surgery and gets home tomorrow.
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Re: Charge rates

Postby jpreou » Tue Aug 23, 2011 9:31 pm

I hope everything goes (went) well with your wife Bruce.
Another question(s); My charger has two balance ports but only one output port. In order to charger two 5000mah 6S batteries at the same time...
  • Do I need a parallel output lead (like the attached pic only with banana plugs at the single end)? (which I can make easily enough myself)
  • Then, would I still choose 5A for a 1C charge to both batteries or do I need to select 10A so each battery gets 5A each?
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Re: Charge rates

Postby RCModelReviews » Tue Aug 23, 2011 11:28 pm

Yes, you can parallel up the main power leads as you describe but it would take just as long to charge two packs in parallel as it would take to charge them separately so I don't know that I'd bother.

Unless you were planning to just hook both batteries up and leave them charging -- which isn't a good idea either. There are plenty of examples of what happens when you leave charging lipos unattended and the results often involve flames, smoke and significant property damage.
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Re: Charge rates

Postby jpreou » Wed Aug 24, 2011 3:44 am

especially if you use gasoline, huh? ;-)
(oh, and thanks!)
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Re: Charge rates

Postby ergocentric » Wed Aug 24, 2011 1:14 pm

RCModelReviews wrote:Yes, you can parallel up the main power leads as you describe but it would take just as long to charge two packs in parallel as it would take to charge them separately so I don't know that I'd bother.

Unless you were planning to just hook both batteries up and leave them charging -- which isn't a good idea either. There are plenty of examples of what happens when you leave charging lipos unattended and the results often involve flames, smoke and significant property damage.


Plus you would really need to parallel the balance ports as well
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Re: Charge rates

Postby jpreou » Wed Aug 24, 2011 7:37 pm

One at a time it is at 1C, or 2C if I'm short on time. I'll have another two of the 5000s arriving from HK soon but it seems on the order tracking site that maybe the plane doesn't want to leave Singapore airport as they have been there for a few days now! ;-)
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Re: Charge rates

Postby ergocentric » Wed Aug 24, 2011 8:27 pm

jpreou wrote:One at a time it is at 1C, or 2C if I'm short on time. I'll have another two of the 5000s arriving from HK soon but it seems on the order tracking site that maybe the plane doesn't want to leave Singapore airport as they have been there for a few days now! ;-)


It's waiting for my batteries to catch up, in the mean tome check out the nice parallel charging adapter they sell (I was surfing the swarm sale)

http://www.hobbyking.com/hobbyking/stor ... 3F9C2BF818

That is a huge number of connectors for $6

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