12volt charger for 640a/h 12volt battery pack

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rubin
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12volt charger for 640a/h 12volt battery pack

Postby rubin » Sun Feb 28, 2010 9:26 pm

Help, I need a cheap high power charger for a 640 a/h 12volt sealed lead acid pack(160ah 6volt blocks series parrallell).It is located in the woods and supplies off grid lighting for a scouts toilet block ,it is supported by solar cells but runs down in the winter .It only gets charged when I can get a generator (2.8kw)up there and hence I need to chunk lots of amps in over the shortest period of time ,any ideas please?

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Jeremy
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Re: 12volt charger for 640a/h 12volt battery pack

Postby Jeremy » Sun Feb 28, 2010 9:55 pm

The very cheapest way I know to get high current, constant voltage, power supplies suitable for charging lead acid batteries is to use some of the cheap Chinese swiched mode power supplies. I have about half a dozen of these that I use for things like charging my LiFePO4 batteries (via a proper BMS) or running a RC outrunner motor that drives my small milling machine. I've found them to be very robust and good value.

The 12 volt 350 watt versions can be manually adjusted to 13.2 volts, just enough to get a sealed lead acid battery up to around float charge voltage. They are available from several different eBay sellers, like this one: http://cgi.ebay.com/New-12V-DC-30A-350W ... 19ba42897c for around $30 to $40 each, including shipping.

You could, I think, parallel a few of these together to get up to the maximum that your generator can deliver. I suspect that an array of, say, 6 of these supplies (~170 amps) at a cost of around £130, would be the cheapest "quick and dirty" high power charging system you could get. It might possibly be better if the battery could be divided into separate sections for charging, as that way each power supply could feed it's own battery.

Jeremy

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ChrisB
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Re: 12volt charger for 640a/h 12volt battery pack

Postby ChrisB » Sun Feb 28, 2010 10:04 pm

Jeremy, I'm pretty sure normally sealed LA's can only be charged at around 5% of their capacity so using the total a/h figure that equates to around 32Amps.
I'm never a big fan of fast charging to be honest.

I wonder if the two 6v strings stay connected in parallel all the time ?? if they do then during long periods of inactivity, like winter, it might be prudent to disconnect the parallel link so they cant start discharging into each other, quite likely that will keep the terminal voltage up a bit more.

ChrisB
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Jeremy
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Re: 12volt charger for 640a/h 12volt battery pack

Postby Jeremy » Mon Mar 01, 2010 7:08 am

The 5 year old sealed lead acid starting battery in my motorcycle is only about 20 Ah, yet gets charged by the alternator at around 25 amps and has a CCA of around 400A, so I think that 0.05 C seems a bit low as a charge limit for sealed batteries in general.

I'm pretty sure that other sealed AGM LA batteries, like Hawkers and Optimas, are also good for at least a 1C charge, aren't they?

The big sealed standby LA batteries used in UPS systems also have a pretty quick re-charge time, I think, as I have a feeling that there is a specified "must be ready for another outage" time in their spec that's only around an hour or so, implying charge rates of around 1C.

I think that the charge limit you may be thinking of applied to small gel batteries, of the type used in burglar alarms, emergency lighting etc as standby batteries, not the AGM sealed batteries that are now commonplace in high power applications. These gel batteries had little ability to absorb excess gas when charging fast, so did have low charge current limits, but I don't think they were ever made in really high capacities.

My guess is that this 640 Ah pack will be made up from sealed AGM batteries, most probably those used in big UPS systems. If so, then charging it at 170 A would only equate to a 0.26 C charge, so not really that fast.

I'm not a fan of fast charging either, but for this application it looks like such a charge would only be occasional, maybe just a few times a year when the solar system can't keep the bank topped up. For user convenience a fast charge is desirable, because I'm assuming that they don't want to stay out in the wilderness minding a generator for 20 hours!

Jeremy

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ChrisB
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Re: 12volt charger for 640a/h 12volt battery pack

Postby ChrisB » Tue Mar 09, 2010 10:46 pm

Jeremy wrote:I think that the charge limit you may be thinking of applied to small gel batteries, of the type used in burglar alarms, emergency lighting etc as standby batteries, not the AGM sealed batteries that are now commonplace in high power applications. These gel batteries had little ability to absorb excess gas when charging fast, so did have low charge current limits, but I don't think they were ever made in really high capacities.


Ah could be where I've remembered that figure from the

ChrisB
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