What limits charging current?

[Bailes1992]

Currently working through a project for my Duster which will require the use of a second deep cycle battery.

The battery I've selected is a small 7.5Ah unit that has a max charging current of just 10A.

I'm trying to work out if I need a B2B charger or if I could get away with a simple relay that comes on with the ignition or a voltage sensitive relay.

So are batteries designed in such a way that if 12-16v is applied to the battery (assuming it is empty), it would only accept the maximum charge rate? I.e. the resistance across the battery low enough when empty that at it's rated voltage, only the maximum current could flow?

I feel like this is an embarrassing question being an electrician by trade, but little experience with batteries.

 

[ka9mnx]

Current is limited by the source (charger). The battery will accept as much current as the supply can supply.

 

[Bailes1992]

Current is limited by the source (charger). The battery will accept as much current as the supply can supply.

Makes sense. Also an absolute shame because it means I need a small B2B charger instead of a cheap £2 relay.

 

[ka9mnx]

Current can also be limited by the battery internal resistance, in a way. With a poorly regulated supply, the supply will supply it's advertised current rating and then some. In a short condition, with a poorly regulated supply, the supply will output it's rated amps and more until it smokes.

 

[Urshurak776]

Also over-voltage limit of each cell in battery.

 

[Thermo1223]

A trailer or breakaway battery charger is what you want.

 

[Bailes1992]

A trailer or breakaway battery charger is what you want.

That's what interests me. We have a caravan which has a 110Ah sealed lead acid battery in it. On the move it is charged by the car via a 20A feed which runs through a voltage sensitive relay and 2.5mm (approx 14AWG) cable direct to the caravan. So what limits the battery charging rate there? The relay is one of these...

RCT465

www.ringautomotive.com

These are about £10 vs £60 for a 9A 12v to 12v battery charger.

I have now decided on a LiFeP04 battery now which I've read won't be a good idea to charge via a smart alternator anyway. So I think these are out regardless.

I'm considering the Victron Orion 12/12v 9A battery charger now.

 

[brianl703]

Currently working through a project for my Duster which will require the use of a second deep cycle battery.

The battery I've selected is a small 7.5Ah unit that has a max charging current of just 10A.

Old cellular "bag phones" used a 1.2Ah 12V sla battery which was charged via a cigarette lighter plug. The only thing in the circuit between the cigarette lighter plug and the sla battery was a diode. That was probably only there to prevent 12V being on the exposed terminals of the cigarette lighter plug (which could short out on something metal).

 

[Bailes1992]

Old cellular "bag phones" used a 1.2Ah 12V sla battery which was charged via a cigarette lighter plug. The only thing in the circuit between the cigarette lighter plug and the sla battery was a diode. That was probably only there to prevent 12V being on the exposed terminals of the cigarette lighter plug (which could short out on something metal).

I was originally considering a 9Ah 12v SLA battery. But the charge rate was a mere 2A.

I'm looking to install a 2kw diesel powered heater to preheat the car on cold frosty mornings. The device I'm planning on installing will draw 3.5A maximum for a period of around 30minutes to an hour while the engine is off. But I'd like something that would top back up fairly quickly. Or maybe I'm over engineering it.

 

[brianl703]

Current is limited by the source (charger). The battery will accept as much current as the supply can supply.

That is not true. The amount of current a battery will accept depends on it's size, state of charge, well as the charge voltage. It is not correct to say that a 7AH 12V SLA connected to a 13.8V, 50 amp supply will pull 50 amps, even if it's deeply discharged.

 

[thastinger]

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[SHOZ]

That's what interests me. We have a caravan which has a 110Ah sealed lead acid battery in it. On the move it is charged by the car via a 20A feed which runs through a voltage sensitive relay and 2.5mm (approx 14AWG) cable direct to the caravan. So what limits the battery charging rate there? The relay is one of these...

RCT465

www.ringautomotive.com

These are about £10 vs £60 for a 9A 12v to 12v battery charger.

I have now decided on a LiFeP04 battery now which I've read won't be a good idea to charge via a smart alternator anyway. So I think these are out regardless.

I'm considering the Victron Orion 12/12v 9A battery charger now.

That just looks like it disconnects the trailer when the engine is off basically. There is some logic about interior lights and charging switching..

 

[brianl703]

I was originally considering a 9Ah 12v SLA battery. But the charge rate was a mere 2A.

If you want quicker charging, you need a higher voltage than what the alternator in a car is designed to supply. But you can't leave that higher voltage applied to the battery once it's fully charged because it can damage the battery. So you really need some sort of multi-stage smart charger.

 

[Thermo1223]

That's what interests me. We have a caravan which has a 110Ah sealed lead acid battery in it. On the move it is charged by the car via a 20A feed which runs through a voltage sensitive relay and 2.5mm (approx 14AWG) cable direct to the caravan. So what limits the battery charging rate there? The relay is one of these...

RCT465

www.ringautomotive.com

These are about £10 vs £60 for a 9A 12v to 12v battery charger.

I have now decided on a LiFeP04 battery now which I've read won't be a good idea to charge via a smart alternator anyway. So I think these are out regardless.

I'm considering the Victron Orion 12/12v 9A battery charger now.

Is it ran of the tow connector? That 20amp is for more than just the battery. Running & signal lights for one. The car also knows based on current draw what voltage to supply I would guess.

 

[Bailes1992]

Is it ran of the tow connector? That 20amp is for more than just the battery. Running & signal lights for one. The car also knows based on current draw what voltage to supply I would guess.

That just looks like it disconnects the trailer when the engine is off basically. There is some logic about interior lights and charging switching..

Similar questions...

The caravan has a series of relays and there's a permanent 12v terminal on the tow plug and a switched 12v terminal on the tow plug.

Essentially the permanent 12v feed powers the fridge and the two relays. The two relays disconnect the battery from the caravan and connect the battery directly to the switched 12v output on the tow plug so the battery can be charged on the move.

 

[ka9mnx]

That is not true. The amount of current a battery will accept depends on it's size, state of charge, well as the charge voltage. It is not correct to say that a 7AH 12V SLA connected to a 13.8V, 50 amp supply will pull 50 amps, even if it's deeply discharged.

I was quoting a worse case scenario. What you are stating is true with a partially charged battery. It's about the internal resistance of the battery at it's charge state.

 

[barryh]

Ultimately charging current is dictated by charging voltage. If the charging voltage exceeds battery voltage then a current will flow. Once a battery reaches full charge then an excessive charging voltage will eventually damage it. That's why modern smart chargers drop to a float voltage of approx. 13.5 volts once the battery has reached full charge.

The exact current that flows for a given charging voltage is determined by the batteries internal resistance and that falls when temperature increases so at high temperatures a lower charging voltage may be applicable. Quality battery chargers have a negative temperature coefficient built in which will lower the charging voltage as temperature rises.

I haven't looked in detail to see if car voltage regulators have a built in negative temperature coefficient but I'd be surprised if they didn't. I know for a fact that the VR on my 44 year old motorcycle has that built in and have have confirmed that the charging voltage drops a few tenths as temperature rises.

I wouldn't worry that your small battery will charge at too high a current as long as the charging voltage is not higher than the battery manufacturer recommends.

 

[SHOZ]

I'd say you still need something to limit the charge when the aux battery is fully charged. May not be the case if there is always a load on it while traveling.

You need something like this.

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www.redarcelectronics.com

 

[eljefino]

A 7.5 amp-hour battery that charges at 10 amps will be done in 45 minutes. That's way different than the ten-hour rule for lead acid batteries. However, like said above, it develops charging resistance as it fills up, and the amps flowing in will slow down in response.

The battery in question, you should pull a spec data sheet, to see what its charge rate is at your alternator voltage, and whether said voltage would damage it if applied forever.

 

[Jetronic]

That's what interests me. We have a caravan which has a 110Ah sealed lead acid battery in it. On the move it is charged by the car via a 20A feed which runs through a voltage sensitive relay and 2.5mm (approx 14AWG) cable direct to the caravan. So what limits the battery charging rate there? The relay is one of these...

RCT465

www.ringautomotive.com

These are about £10 vs £60 for a 9A 12v to 12v battery charger.

I have now decided on a LiFeP04 battery now which I've read won't be a good idea to charge via a smart alternator anyway. So I think these are out regardless.

I'm considering the Victron Orion 12/12v 9A battery charger now.

internal and wiring resistance limits the charge current, together with the limited alternator voltage. If your alternator is supplying 14.4V, the battery will see less. And the more current it draws, the less the voltage will become.