What rate does the Alternator charge the leisure battery

[davidathomas42]

So. I have previously put a voltmeter on a vehicle battery before in the engine compartment and seen it pulling 30-35amps an hour...

But what rate can the alternator throw power into the leisure battery. And how does it do it?

Does it go direct to the battery or via the habitation electrical equipment. if so what rate can it throw power into it.

I ask because I am still mainly off grig camping with just one 125amp leisure battery. Its normally grand for the 2 days I use it and stays in the green of 12.5v area. but this week it was in the amber display with 11.5v showing off load. Never normally gets that low. And with just 90mins driving it was quickly back to that level again.

I have now had it on EHU for 2 days to smash it full properly. But was unsure how much driving it would need to throw power back to full as I have no idea the charge rate. Guess I could hook the amp meter up to it with the engine on. But still nice to know how the power gets to it.

 

[Richard n Ann]

Cheapest way is to buy a clamp meter.



Got one of these myself, very handy with full multimeter ranges too

 

[pappajohn]

Invariably the split charge wiring is woefully undersized and doesn't allow much current to pass.
On top of that, as the engine battery reaches full charge after starting the alternator output drops off dramatically so the leisure battery may only be getting a few amps at best regardless of whether its direct of via the control circuits.
The battery voltage may show 14v plus but it may only be a couple of amps going in.

A B2B charger replaces the split charge circuit and fools the alternator into seeing a discharged engine battery so ups the output even if the starter battery is fully charged.
That current then goes to the hab battery at 30amps or 60amps depending which B2B model you have.
It's all controlled electronically so doesn't overcharge or discharge when the engines not running.

 

[Minxy]

@pappajohn

As there are B2B (battery to battery) and A2B (alternator to battery) chargers, which would be the best scenario for giving the most input to a leisure battery?

 

[Wissel]

An Alternator "can" charge at whatever it's maximum output is. Although if it did for long, it wouldn't last long (unless a specialised alternator designed to be run at full capacity - mainly used in boats I believe).

But it isn't the alternator that decided how much charge to send to the leisure battery, the battery decided how much to take.

How much charge a leisure battery takes will depend on what battery it is. Some will charge at about 0.1C (C is the battery capacity, so in your case 0.1C is 12.5A). Some charge at 0.2-0.3C (some AGM and deep cycle I believe), and at the other end of the scale, some LiFePO4 can charge at 3C.

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[Lenny HB]

Depends on the alternator and the split charge wiring, like most German vans my Hymer has an uprated alternator and 16mm sq cables on the split charge wiring.
If the batteries are fairly flat can get a charge rate of over 40 amps for a short while.
In normal use starts charging at around 20/22 amps then as the battery voltage comes up after 10 - 15 mins it drops to about 14 amps.

On Brit built vans the wiring is wofully undersized and will start around 10/12 amps and drop to 4/5 amps.

I ask because I am still mainly off grig camping with just one 125amp leisure battery. Its normally grand for the 2 days I use it and stays in the green of 12.5v area. but this week it was in the amber display with 11.5v showing off load. Never normally gets that low. And with just 90mins driving it was quickly back to that level again.

You are seeing the alternator voltage (after the volt drop on the cables), it will take around 500 miles driving to fully charge the battery.

 

[Bill and Tracey]

Is the battery a wet lead acid? Have you checked the levels? How old is the battery? Might be worth upgrading the wiring as previously mentioned by other members. I have run 4 gauge audio power cable throughout my charging system and wired in several extra earth points around the engine bay and along the chassis.

 

[pappajohn]

@pappajohn

As there are B2B (battery to battery) and A2B (alternator to battery) chargers, which would be the best scenario for giving the most input to a leisure battery?

I have no idea but both will be far superior to any split charge system.