I’ve just received a new battery charger for car batteries in the post. It shows max 20-55ah range for this charger, but my battery is 100ah.
What will happen if I attempt to charge the battery?
(googling this just gives sites selling batteries :mad: )
While I’m familiar with car batteries being rated in ampere-hours, I’m not familiar with battery chargers being rated that way. I see them rated in amperes (presumably we’re talking about 12 volts across the board here). A charger with a higher ampere rating can charge a battery faster than one with a lower rating. Many have a selector switch for high and low charging rates, and some also have a “start” function (extra-high rate) to allow cranking the starter before the battery is fully charged. Typically a low rate is 2 amps, high is 10-15, and start is (not sure here) about 50.
It’s generally safest to charge a battery at a slower rate. Charging at a high rate can sometimes help shorten the life of a battery that could have been saved if it had been charged slowly. Knowing that, I would assume that your charger may be preferable to one with a higher rate. Still, I’d like to know how this ah rating compares to the amp rating that I normally see.
The charger will charge between 4.5 and 7amps. I think the 100ah refers to the total capacity of the battery and the 20-55 of the charger refers to the range of battery ah it is suitable for - not the actual rate it charges at.
It might be an international thing - car batteries seem to commonly have an ah rating.
I’ve never heard of a car battery charger having a rating of ah. It really doesn’t make sense.
What’s the brand and model #?
In its primordial form, an automotive battery charger is just a big transformer and a couple of diodes. They don’t care whether they’re charging a big battery or a small one.
It’s not the charger’s own rating. It’s a statement that the charger is to be used with batteries with a minimum of 20 and maximum of 55.
It’s an ‘automatic’ charger and also a tester. So it stops full charge when it detects the battery is full.
Well, I’m confused. I’ve bought and used a number of different automotive battery chargers over the last few decades and until now never even heard ampere hours mentioned in connection with them. Offhand, I can’t see where the battery’s ah rating would be of concern other than as a guide to how long it might take to reach full charge. And while some car batteries are rated at less than 55 ah, I’ve never heard of any with ratings that even approach 20 ah. Many are in the 60-100 range, which raises the question of just where could one use a charger with the limitation described.
What is the source of this 20-55 ah statement?
Looking at the specs provided in the link, you can charge any 12 volt auto battery you want. Don’t worry. It’ll work good. Enjoy your purchase.
http://www.drapertools.com/products/manuals/66800ins.pdf
scroll to page 7
edit: I did try it… all the lights came on as if battery full, so I am not convinced. I borrowed a more basic charger from someone and the battery is now charging.
Right. A charger with an amp-hour rating is like a pump that says “For filling tanks between 20 and 55 liters.” Such a pump will certainly fill a 100-liter tank - it will simply require a bit more time to do so.
Provided the voltage is correct and the current is within the range the battery can safely accept, there should not be a problem using the charger with any battery.
Though the manual doesn’t make it clear, it sounds as if this charger may have a sensing circuit that applies a load to the battery and watches what happens to the battery voltage. A large-capacity battery may thus test out as fully charged when it could accept more charging. (A slightly more sophisticated circuit could easily deal with this.)
Hmm. This is an interesting conundrum. While I have plenty of experience using battery chargers, I don’t claim to be an expert on their design. I will offer my “common sense” take on the matter, with the disclaimer that maybe there’s something significant that I don’t know.
My thought is that the only practical difference the battery’s ah rating makes is how long it takes to fully charge the battery from a given point (e.g. 25% discharged). If that’s true, then the only affect on the charger would be how long it charges at the higher rate before it goes into trickle-charge mode. With a good quality charger, I wouldn’t expect that to matter. Further, I would think the charger would operate essentially the same in charging a 50 ah battery that’s 20% discharged as in charging a 100 ah battery that’s 10% discharged.
It’s hard for me to imagine any way the battery could be harmed. It’s conceivable the charger could be stressed and possibly harmed, but as I mentioned before, I’ve just never heard of such before. Of course, now I’m curious as to just why the instructions include that recommendation. Any engineers from Draper on the boards? 
ETA: Somehow I completely missed the part about the charger indicating full charge when apparently that’s not the case with this battery. That sounds like a plausible expanation for the ah spec.
Nothing useful to contribute. I do want to tell you, though, that I was trying to figure out what the hesitation over “charger” and “car” was all about. I was looking for some kind of double entendre.
-FrL-
[deadpan]You are funny[/deadpan]
(sarcasm aside: 
I liked the humour)
I am sure this obvious by now but, anyway, the analogy is more like that it is a pump that will stop pumping when it has reached 55 litres, even though the tank capacity is 100 litres.
reply to all: Using the borrowed charger I charged for about seventeen hours, put the car back in the car, and it worked… an immense releif.
Now contemplating whether it is worth the hassle and postage to return a £28 charger (£22 + p&p) back to amazon.
It’s a bit more complicated than that because the tank nearly always starts out partly full, and the pump has no direct way to learn what was the starting volume.