Testing cheap 40 amp chargers

Submitted: Wednesday, Jul 25, 2012 at 23:02
ThreadID: 97096 Views:6423 Replies:2 FollowUps:11
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Recently I was looking for a 75 to 100 amp battery charger to fast charge a couple of batteries in my caravan. I got a bit of a shock at prices around a $1000.
I opted for a 2 cheap 40 amp models (1 for each battery) on Ebay at around $90 each.
They are advertised as a 3 stage – bulk, absorption and float stages.
I brought one and after testing found:
It does charge at 40 amps whilst only drawing 3 amps (RMS) on the 250 volt input.
It’s light and compact at 2.3 Kg.
It’s definitely not 3 stage - it puts out about 12.5 volts at 40 amps which rises to 14.5 volts when the charge rate tapers off to 1 amp. Then it stays at 14.5 volt – not good for a float.

It will suit me and I have ordered a second one.
I will switch them in when only needing a fast charge.
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Reply By: Sand Man (SA) - Thursday, Jul 26, 2012 at 07:40

Thursday, Jul 26, 2012 at 07:40
Dennis,

40 Amps is way too much for a couple of batteries.
And cheap battery chargers may well destroy your batteries and you will be up for a high replacement cost.

What you need is a "smart" charger of between 15 and 25 amps.
You also don't need two chargers. Your batteries in the caravan should be connected in parallel and in charging one, you will be charging both.

From the Ctek advertising statement, "The Ctek MXS 25 is a fully automatic 8-step charger that delivers 25A to 12V batteries from 40–500Ah.
The charger is IP 44 classified (outdoor use), designed to protect vehicle electronics and non-sparking, reverse polarity protected and short-circuit proof. It is delivered with a 2-year warranty."

Bill


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Follow Up By: Dennis Ellery - Thursday, Jul 26, 2012 at 10:07

Thursday, Jul 26, 2012 at 10:07
Hi Sandman,
As the old saying goes "Oils aint oils Sol"
Im going to use 80 not 40 amps - your theory only applies to lesser quality batteries – in fact 80 amps is more beneficial to the batteries I am using. A 15 to 25 amp charger would only shorten thier life.
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Follow Up By: Ross M - Thursday, Jul 26, 2012 at 11:15

Thursday, Jul 26, 2012 at 11:15
Unless you are using Nicad batteries then it seems 40amps into one battery is far too much. eg 120ah AGM is max at 20 amps.

So, are you using some batteries that no one else has access to and they can actually receive/absorb 80 amps into two batteries and survive???
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Follow Up By: Dennis Ellery - Thursday, Jul 26, 2012 at 11:28

Thursday, Jul 26, 2012 at 11:28
Hi Ross,
I am talking about AGM's - you need to do a bit of research.

A lot of batteries are limited to C/5 rate maximum charge (20amps into a 100amp hour battery) and benefit from slow charging rates

Lifeline batteries are just the opposite and quote that rate as a minimum, the following is an extract from their Technical Manual 6-0101.

“The charging current during the Bulk stage should be set as high as practical; higher current levels mean faster recharge time. For repetitive deep cycling, chargers should have an output current of at least 0.2C (20 Amps for a 100 Ah battery). If the output current is less than this value, the cycle life of the battery may be negatively affected. Due to the low impedance design, Lifeline® batteries can tolerate in-rush current levels as high as 5C (500A for a 100Ah battery)”

It’s not just Lifeline but a number of other top of the line batteries can accept fast recharges without detriment – Optima quotes figures of 100amps into their 75amp hour deep cycle - and there are others if you want to do the research.
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Follow Up By: Sand Man (SA) - Thursday, Jul 26, 2012 at 18:59

Thursday, Jul 26, 2012 at 18:59
Dennis,

I'm not going to talk you out of anything, but 80 amps is over the top in my opinion.

You seem rather obsessed with high charging current to get a full charge as soon as possible and yet you intend using two "cheap battery chargers".

Good luck mate.

My 7 amp Ctek manages to fully charge my two 80Ah AGM batteries in my camper,
but I don't care if it takes a day or two to achieve it. I only use the AC charger at home, after a trip to apply a maintenance charging process. They are rarely depleted below 11.6 volts as the onboard Waeco has built-in low voltage protection. They are also charged while driving via a DC-DC multi-stage charger, so the batteries are maintained in good order.
If I needed to reach a full charge quicker, I would use the on-board charger which has a selectable current range up to 30 amps, but I have never needed to use it.
I don't camp in caravan parks and don't use a generator either.

Oh, and full charging of the batteries can only achieved when the charger has reached the "maintenance" or pulse mode of the multi-stage charging process.





Bill


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Follow Up By: Dennis Ellery - Thursday, Jul 26, 2012 at 21:28

Thursday, Jul 26, 2012 at 21:28
Hi Sandman - you do things differently to me.
I stick to the manufacturer’s specifications.
These two cheap chargers will fast charge to Lifelines specs but not to their float requirements.
When not needing a fast charge I will switch the batteries to an existing 15 amp battery charger that will float charge to Lifelines specification.
Lifeline doesn’t require a 5, 7 or 8 stage charging system to maintain their batteries – possibly due to the quality of the materials they use. You should check their technical manual.

“Obsessed with high charging current to get a full charge as soon as possible” you are dead right.
In a number of camps I have been in lately there have been campers running their gensets for 5 or 6 hours a day often after 8pm – people find this irritating.
If I need to charge my batteries I want to run the genset less than an hour, at a reasonable time – between 9am and 3 pm.
Yes I do have solar panels but they don’t keep up all the time.
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Reply By: Cravenhaven - Friday, Jul 27, 2012 at 12:21

Friday, Jul 27, 2012 at 12:21
Care to post a link to the items on ebay. I agree with your sentiments about wacking in a high charge for a short time while you have access to mains power from the likes of a generator etc.
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Follow Up By: Dennis Ellery - Friday, Jul 27, 2012 at 19:19

Friday, Jul 27, 2012 at 19:19
Hi Cravenhaven
I assume you know your battery’s capacity.
If it’s an ordinary 100amp A/H AGM or Gel you could damage it.

http://www.ebay.com.au/itm/NEW-12V-Battery-Charger-40-Amp-Car-Boat-Caravan-/280853947994?pt=AU_Car_Parts_Accessories&hash=item416433265a
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Follow Up By: Cravenhaven - Friday, Jul 27, 2012 at 20:40

Friday, Jul 27, 2012 at 20:40
I want one for the times we only have a short amount of charging time and a low battery as I have a 7amp CTEK for other times. From what you said it acts as a basic charger and not a constant current source and actually delivers the rated current so it will do me.
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Follow Up By: Dennis Ellery - Friday, Jul 27, 2012 at 23:31

Friday, Jul 27, 2012 at 23:31
Yes it’s a pretty basic charger – puts in 40 amps when the battery’s flat – tappers off to 14.5 volts and under 1 amp after the battery fills up. I wouldn’t leave it connected long term as it would shorten the life of the battery due to no float stage.
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Follow Up By: oldtrack123 - Saturday, Jul 28, 2012 at 11:39

Saturday, Jul 28, 2012 at 11:39
Hi
I think that is what should be understood
They are "Constant voltage chargers"

The current will be high for a battery with low SOC & will gradualy decrease as the battery SOC rises .
Some, by design, can have near constant current[up to a limit]

Peter
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Follow Up By: Dennis Ellery - Saturday, Jul 28, 2012 at 12:06

Saturday, Jul 28, 2012 at 12:06
That’s correct.
I purchased and tested one first - found it ideal - then purchased a second one.
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Follow Up By: Dennis Ellery - Saturday, Jul 28, 2012 at 13:58

Saturday, Jul 28, 2012 at 13:58
This may help
The following is from my load test graph of a partially discharged battery.
Start 30 amp at 13.8v
After 15 min dropped to 20 amp at 14.05v
Another 15 min dropped to 12.5 amp at 14.20v
Another 15 min dropped to 10 amp at 14.25v
3 hours later dropped to 1 amp at 14.48v

The charge rate dropped of rather quickly as I guess the battery was only about 20% DOD - A 50% DOD wouldn't have dropped off so quickly.
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