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Charging a battery

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cobra1

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Hi guys,

I am working on a an idea at the moment and am stuck on something.

If i had a bank of batteries, 6 x 12v@120AH wired in series to give me a total output of 72v @ 120AH

This is a total of 8640W or for sake of this discussion 8Kw (please correct if this is wrong)

If i were to use these batteries until they were at 50% capacity, and then start to charge them with a 2.5kw power source, how long would it take to charge these batteries??

also can a battery be charged at the same time as it is being used??
 
I'll let someone else answer the charging question. Don't know what the efficiency of power in to battery charge is. I do know that you normally charge batteries at the C/10 or C/20 rates, which would be 800 or 400 watts in. 2.5Kw would charge too quickly.

But you can use a battery while charging it. Although technically at that point you're not using the battery anymore, you're taking power directly from the charger. The voltage is going to be higher when charging, so make sure you take that into consideration.
 
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thanks for the reply smanches, i guessed it might draw power from the charger instead of the battery, i might be wrong but would i be correct in saying that if for arguments sake i was supplying the batteries with a given voltage 90v for instance and the load was using 72v the remaining 18v would go into the battery??
 
Ok let me take a stab at this...

Mathematically, the batteries are rated at 120Ah. The charge rate based on C/10 would be 12 amps or 6 amps for C/20.

If the battery is discharged to 50%, then it would take 60/12 5 hours at the C/10 rate or 60/6, 10 hours at the C/20 rate.

These calcs dont take into account charging efficiency or battery internal resistance, this is just a theoretical example.

The charger would need to have an output voltage that is a function of the battery's chemistry...SLA batteries as an example use 2.3v/cell or 13.8v as the charging voltage, it also needs to be current limited to avoid boiling the electrolyte by too fast a charge current.

If your batteries are of the SLA type then your charger would need to output 82.8 volts at 6/12 amps...close to 1kw for the 12 amp rate...Your 2.5kw is a little on the large side and doesnt tell me what its capabilities are in terms of voltage...
 
The output voltage is mains voltage 240v or i can use 110v. upto 2.5kw i havnt calculated current but i would imagine 13A would be easily achievable.

is it possible to charge a lead acid battery in less than 10 hours?? would be aiming towards say 2-4 hours charge time if possible.
 
Recharge time is dependant upon the battery's internal resistance to some degree....

I wouldnt have thought a LA battery would cope with fast charge rates, but then I'm no expert...

That said, if the C/10 rate is 12 amps and would take 5 hours, you could go to C/n to get the required charge time...

Depends on how long you want the batteries to last...fast charging is sure going to reduce their longevity...

Is having a second set of battteries an option? One set on line while the other recharges?
 
a second set of batteries is not really practical, heres how the setup goes

these batteries are in an electric vehicle, powering an electric motor, the idea is to use the batteries to drive the car and then for a fuel cell to charge the batteries at night time ready for the following day.
volvo are working on a similar concept, it uses batteries, but when they become low on power an engine starts that will charge the batteries and run the motors until the batteries are charged again.

i plan on using a fuel cell though instead of a petrol powered engine though.


for this idea to practically work i think it would need a recharge time of around 4 hours, faster if possible, this means that you could use the car to go to work, whilst at work the car charges itself, then you go home and it charges itself overnight, you might go to friends house, while your there it charges itself, etc etc.

i estimate that the bank of batteries to last for about 100 miles on a full charge, i think that if you can do 100 miles every 4 hours without actually plugging the car into the mains this would be progress
 
You are not going to do this with off the self lead-acid batteries, sealed or flooded. It will take batteries like used in the Prius; about $8000 per set.
 
If i had a bank of batteries, 6 x 12v@120AH wired in series to give me a total output of 72v @ 120AH

This is a total of 8640W or for sake of this discussion 8Kw (please correct if this is wrong)
That's 8640Wh or 8KWh. If you draw 120A from the battery at 72V, that would be 8640W.
 
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yes i know it is, i was saying 8kw to make it easier.

i have been reading the link above and have come to the conclusion that lithium ion batteries are much better for me, although they do cost a small fortune.
 
OK. It wasn't clear that you understood the difference between kw (power) and kwh (energy).
 
You can use the batteries and charge them at the same time. You have to use several diodes to do this.

Do not charge the batteries too fast the acid will boil and over heat and distroy the battery.
 
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