I have a 17.5 amp hour gel cell battery and a small charger designed for 7 amp hour batteries. Can it be used to charge the battery, or will it destroy the charger? I am ok with it charging slowly since the battery is rarely used.

 

It depends on the charger. Most will only provide a charging current around their maximum.

However, not charging a lead acid battery "hard" enough will result in sulfation which will reduce battery life.

 

What is its maximum charging current?

I would expect that a charger designed for a 7Ah battery would readily charge a 17.5Ah battery.
It will just take about 2 1/2 times as long.
It should still charge it sufficiently "hard" so that it won't sulfate.

 

What is its maximum charging current?

I would expect that a charger designed for a 7Ah battery would readily charge a 17.5Ah battery.
It will just take about 2 1/2 times as long.
It should still charge it sufficiently "hard" so that it won't sulfate.

Agreed.

 

I have a 17.5 amp hour gel cell battery and a small charger designed for 7 amp hour batteries. Can it be used to charge the battery, or will it destroy the charger? I am ok with it charging slowly since the battery is rarely used.

There's various types of charger that might behave differently.

My instinct would be to try it, but keep an eye on it to make sure nothing runs hot.

 

It says it puts out 1.3 amps. I will give it a go and see how it handles it.

 

I tried charging it and the charger stopped working. It didn't fail immediately. I watched it for a bit, went to eat, came back, and it was dead. I opened it up and was greeted by a burnt component that looks like it may have been a capacitor judging from the markings on the board. Since it's though hole, a repair would normally be quite easy, but since the component is burnt to a crisp, I doubt it will be easy (or possible for that matter) to figure out what specific component is needed. The specific charger model is the yaso-13815. The odd thing is that I took a closer look on the back of the casing and it looks like the charger should work fine with my battery. Here is a picture of the back of the casing that I found online and a picture of the damaged board. I can't read the back of the casing entirely, but it looks like it's supposed to be some table that tells how long a battery will take to charge.

 

That burnt component on the lower left of the board is likely a power resistor.
Not sure how to determine its value.

 

burnt component that looks like it may have been a capacitor judging from the markings on the board

Looks like a resistor to me and there's more than just one burned component:


Can you trace the components and make a schematic?

 

I will do my best to draw out the circuit, but I won't have it done for a day or two. I have some exams to prepare for. One of the busted components appears to have tons of solder on one of it's leads. Not sure how that would have happened since (I'm pretty sure) the board was facing up when it was charging. I will try to figure out what it is when I get home later today.

 

A big battery can require a good deal of current for a long time and many switchmode converters lack accurate current limiting. In consequence they don't protect themselves from overload. Even a small battery that had a shorted cell could overload a badly designed charger. There are some integrated off-line switcher ICs around now that do a pretty decent job of protecting both themselves and the output load from excessive current.

I suspect the large resistor in the lower left corner is actually at least sort of intended to be a fuse. It probably is between one of the wires to the AC input and the bridge rectifier. I seems it sort of worked like a fuse without starting a fire. I think the likely sequence of failure was the device mounted on the heatsink (power bipolar transistor or power MOSFET), small visibly damaged devices and then the pseudofuse.

I don't see any safety agency markings on the label, which is not unusual for such products from China. I adopted a policy several years ago that I simply will not use such things. I've seen too many of them that would be refused certification by every safety agency on the planet because they are fire and/or electrical shock hazards. That one probably would also fail EMI/RFI standards.

My advise is not to waste time trying to repair it.

 

I just noticed that what appears to have been a TO-92 transistor, Q2?, looks like it has detonated. Light emitting transistors are briefly entertaining.

 

The part that broke off would probably explain the rattling noise that I was hearing. I also just ran a picture of the back plate through google translate and the back is a table showing how long it will take for a battery to fully charge based on capacity. Since 15-25 amp hour batteries are mentioned on the table, and my battery is in that range (about 17 amp hours), I think I will contact the seller and see if the charger I got was out of spec or something like that.

 

Do you guys think that the charger was destroyed by over current or by inadequate cooling?