Electronics and battery charging

I have a flat car battery and I have a lithium battery charger (42.5V DC, 2A for charging a 36V battery), I would like to pair the two of them up rather than spend £40 on a charger I'll probably only use once.
I have a Maplin store round the corner and a rudimentary knowledge of electronics (a bit more than that really but it was a long time ago). I would like to use a voltage regulator to bring the 42.5V DC down to about 15V and connect to the battery.
I know enough to understand it's probably not that simple and I may need an extra component or two on my breadboard, can anyone advise?

ISTR that lithium chargers are constant current, but lead acid chargers aren't.

I may be wrong though.




Dead helpful me.

Step away from the soon-to-be-burning plastic and acid magic box.

Originally Posted by Loz

ISTR that lithium chargers are constant current, but lead acid chargers aren't.

I may be wrong though.




Dead helpful me.

That's right for a better lead acid charger, it uses high voltage for the first 80% and then switches to a lower one that won't overcharge it. I'm not too fussed about switching the voltage as I don't need to top it right up, just enough to get me around OK. When I have more time I may set the voltage to the lower setting and leave it running overnight.

Originally Posted by Kebab The Cat

Step away from the soon-to-be-burning plastic and acid magic box.

That would be sensible, that's not me.

What you're suggesting should, in principle, work.

The key thing to remember is the charging voltage is critical. To effectively charge, you need at least 13.8V. But go above 14.4V and you'll cause irreversable damage to the chemical structure of the battery.

Originally Posted by Discharge

What you're suggesting should, in principle, work.

The key thing to remember is the charging voltage is critical. To effectively charge, you need at least 13.8V. But go above 14.4V and you'll cause irreversable damage to the chemical structure of the battery.

Hmmm, a charger in Halfords is £27 and I think the components to make what I want work wouldn't leave much change so I'm going to go with the sensible idea. I'm admitting defeat.

Originally Posted by Mussels

Hmmm, a charger in Halfords is £27 and I think the components to make what I want work wouldn't leave much change so I'm going to go with the sensible idea. I'm admitting defeat.

It's for the best.

Originally Posted by Loz

It's for the best.

Even if best is boring and unadventurous. I've got it now and feel like a failure.

You could just simply charge 3 batteries in series, you would not need to modify your power supply then.

Originally Posted by go_slow

You could just simply charge 3 batteries in series, you would not need to modify your power supply then.

Have you got two flat batteries I can borrow?

Originally Posted by Mussels

Have you got two flat batteries I can borrow?

No but you could try a garage, the tip or freecycle?

Originally Posted by Mussels

Even if best is boring and unadventurous. I've got it now and feel like a failure.

So go invent something else! Maybe something that involves less hot acid and explosions?

Originally Posted by Loz

So go invent something else! Maybe something that involves less hot acid and explosions?

I had to return the charger as the instructions said it was unsuitable for sealed batteries so I may still construct something.
Putting batteries in series won't work as they aren't matched, an uneven charge is not good.
Fortunately I had a couple of hours testing the unsuitable charger before I returned it so hopefully I have enough in there now and won't need anything.