What size charger do I need?

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4Springs
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What size charger do I need?

Post by 4Springs »

I will have a bank of 12 x 12V 140Ah AGM lead-acid batteries. 12x12 = 144V. 144x140 = 20kWh.
I will need to charge my vehicle in about 8 hours. But it won’t be fully discharged at that time. I’m guessing, but I think it’ll be about 50% discharged.
So I need to charge 70Ah over 8 hours.
70/8 = 9 Amps for 8 hours. So I need a 9 Amp charger. Easy!

But is it this simple? I think not.
(Good) chargers have multiple stages, so I presume a “10 Amp” charger would only put in 10 Amps during the bulk charging stage (I think that is the name?). Presumably it would then drop off, and put much less in after that. Also, there would be inefficiencies, especially because the batteries are being charged very quickly, so some of the current would be lost.

So what size charger do I need to charge my battery in the allocated time?

antiscab
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What size charger do I need?

Post by antiscab »

10A charger should be big enough.

for an EV application, 70Ah is the absolute limit of how much you can get out of a "140Ah" AGM battery.

Matt
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

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4Springs
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What size charger do I need?

Post by 4Springs »

Yes, I'm working on the calculation here - (written by Matt!) which gives me a theoretical range of about 60km. So driving 30km into work for the day should use half my charge?

The C20 value of the battery is 140Ah. So if I have travelled 30km, presumably this means that there is a C20 value of 70Ah left in the battery. (although really I've only used 35Ah, because I've used it at about C1)
The other consideration is that I may not need to charge the battery completely full before I make the trip home again. I think that the chargers put in most of their charge early in the cycle? Which means that the 8A charger that Matt offered secondhand might be ok after all. If the 8A charger put in enough to take the battery to (say) 80 or 90% charge, I'd still have enough to get home on. It could then go on a nice long charge overnight, ready for the next day...

Sounds good to me, but I have no idea if my assumptions are correct or not! Image

Cheers,
Christopher.
Last edited by 4Springs on Fri, 11 Feb 2011, 16:19, edited 1 time in total.

antiscab
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What size charger do I need?

Post by antiscab »

actually, its a little more complicated than that.

if you have a 140Ah AGM battery, and used 35Ah in an hour, then let it sit for 8 hours, you end up with remaining capacity somewhere around the C/5 capacity - the 35Ah you have used.

with flooded batteries, its more like the C/10 capacity - 35Ah.

but if you always limit discharge to 70Ah, you're always on the safe side :)

Matt
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

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4Springs
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What size charger do I need?

Post by 4Springs »

Ok, I've looked your reply for ages Matt, trying to figure this out. See if I have it correct.
antiscab wrote: if you have a 140Ah AGM battery, and used 35Ah in an hour, then let it sit for 8 hours, you end up with remaining capacity somewhere around the C/5 capacity - the 35Ah you have used.
When you say the remaining "capacity", are you talking what is in the battery (capacity for using), or what can be put in (capacity for charging)?
So the former has 35Ah left in the battery, and would take 105Ah to charge it back up.
I'm hoping you mean the latter, in which case I won't use so much energy charging it back up (to capacity!). In this case I'll get away with a smaller charger than I was thinking.
antiscab wrote:with flooded batteries, its more like the C/10 capacity - 35Ah.
This bit I don't understand at all. Did you mean 70Ah?

Thanks,
Christohper.


antiscab
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What size charger do I need?

Post by antiscab »

remaining capacity is the amount of energy you can still use.

so you pull out 35Ah in half an hour (~70A continuous).
then you leave it to sit for 8 hours.
after 8 hours you can pull another ~50Ah for a total of 85Ah.
where as if you drew 70A continuous with no rest period, you would only be able to get 70Ah total.

floodeds do better at this.

the amount of Ah you need to put back in = discharged AH * 1.x + a fixed amount.

how much that those values are you won't really know until your battery is in service.

Matt
Matt
2017 Renault zoe - 25'000km
2007 vectrix - 156'000km
1998 prius - needs Batt
1999 Prius - needs batt
2000 prius - has 200 x headway 38120 cells

Jacmille
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What size charger do I need?

Post by Jacmille »

The fact is that the size of charger required depends upon too many variables for there to exist.you have to weigh up various different aspects such as required charge time, expected battery life and insistence on obtaining a full charge in order to know the charger you need.

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Richo
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What size charger do I need?

Post by Richo »

Nop.
Just pay the extra $200 and get the bigger one.

You can spend all day calculting the fifth differential to help decide the exact charger.
But one day you wish you had spent the extra $200 while you wait for a tow after you left work early.
Not to mention cursing you wasted a day calculating the 5th differential on the assumption this would never happen...
So the short answer is NO but the long answer is YES.
Help prevent road rage - get outta my way!

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