12v battery, 18v magic

[TinkerPhil]

I bought an 18v mower with no battery.
Maybe naively I thought I'd be able to run it off a 12V battery charger but no... Nothing!

So I checked all the circuits and they're good weirdly though the 12v input from the charger measured at 18v as if the circuit was so eager for 18v it was forcing 18v back to the source.
When I took the source away the 18v slowly dwindled down

I'm guessing there's some sort of boost circuit in the mower that needs a healthy 18v source to actually power the mower

Now I carelessly ran 12v the wrong way into the circuit - will I have fried it?

so here's a picture of the circuit in the mower

Can anyone give any quidance?

Thanks

 

[Diver300]

An 18 V battery mower will almost always cut off at some voltage to protect the battery, and I would expect that voltage to be somewhere around 14 to 16 V.

A 12 V battery charger will often put out 18 V with no load. Chargers designed to connect directly to a battery are unlikely to be suitable to run anything directly.

The current that the mower will take, especially when starting, will be quite big and you would need a very large power supply to run it.

Connecting a power supply backwards may have damaged something but without a lot more information it's hard to know for sure.

 

[TinkerPhil]

Thanks for the reply

It was actually a car battery I wired in the wrong way round - doh!

I suspect it is fried

but I will try to wire it up with an 18v source and see where that takes me

 

[Nigel Goodwin]

Thanks for the reply

It was actually a car battery I wired in the wrong way round - doh!

I suspect it is fried

but I will try to wire it up with an 18v source and see where that takes me

The proper battery will be Li-Ion (and almost certainly 18650's) - and will be 5 in series (or more than one set of five, in parallel for higher capacity) - personally I'd call it a 21V battery pack, as when fully charged it will read 21V (5 x 4.2V). The 18V figure is when they are flat, 5 x 3.6V = 18V.

 

[TinkerPhil]

Yeah, I was hoping to avoid having to get a converter for my other "18V" batteries and just whack a car battery on it and use it for hours

seems like I will have to go the extra mile and use the proper voltage/capacity batteries

hopefully I've not fried it!

 

[schmitt trigger]

A 12 V battery charger will often put out 18 V with no load. Chargers designed to connect directly to a battery are unlikely to be suitable to run anything directly.

This, exactly this.
Battery chargers are not power supplies.
For a number of reasons.

 

[TinkerPhil]

In my defence this basic battery charger drove my 18v circular saw ok

and I did the original frying using a car battery :-(

I don't believe the charger really wants to pump out 18v, I reckon the driven circuit is somehow raising the voltage itself

there must be some capacitors in there as the voltage tailed off slowly when I removed the charger - hence the "magic"

My question (other than have I fried it) was really are there circuits that take a varying load in and regulate that input to a standard value.

LOL I'm really no good at phrasing this question sorry!

 

[Nigel Goodwin]

In my defence this basic battery charger drove my 18v circular saw ok

and I did the original frying using a car battery :-(

I don't believe the charger really wants to pump out 18v, I reckon the driven circuit is somehow raising the voltage itself

there must be some capacitors in there as the voltage tailed off slowly when I removed the charger - hence the "magic"

My question (other than have I fried it) was really are there circuits that take a varying load in and regulate that input to a standard value.

LOL I'm really no good at phrasing this question sorry!

There's no 'magic', it runs off a 21V (18V when flat) Li-Ion battery, they may a capacitor inside?, but not a very big one, so it wouldn't 'tail off slowly' if was working.

BTW, you even show a (small) capacitor in your pictures.