Does an alternator fully charge a lead acid leisure battery?


Auckland, NZ
Nov 8, 2019
Auckland, New Zealand
A lot of camper vans in the UK use a simple split charge system where their leisure battery (AGM or similar) is connected to their alternator via a VSR (voltage sensitive relay)

I think I read an article once that said that this method of charging was only likely to charge a lead acid battery to 80(ish)% due to the way an alternator works.

I know from MPPT charge profiles that a lead acid battery requires bulk charge, absorb charge and float charge phases to fully charge. How does the charge delivered by an alternator compare with these MPPT charge profiles and does it provide the right kind of delivery to take a leisure battery up to 100% SOC?

What affect would cable resistance voltage drop cause too?

I'm not planning to use a split charge system, I'm happy with my solar plan. I'm just wanting to learn and work out the nuances with this particular charging method




Solar Addict
Jul 24, 2021
Automotive alternators are designed to recharge a healthy lead acid battery briefly after starting along with being able to support the lighting, ventilation and other loads applicable to the vehicles operation. Whilst it is possible to charge auxillary batteries it will reduce the alternators life expectancy due to the additional demand.

Typically deep cycle batteries are used in the application described which need different characteristics which the alternator is not designed to replicate so the battery will not receive the requirements needed, possibly resulting in shorter lifespan.

Additional loading especially with large capacity banks can cause the alternator to overheat, lithium batteries, even of lower capacity, are of such low internal resistance they are a virtual dead short which will cause rapid alternator failure if there's no current regulation.

Voltage drop is dependent on current, conductor size along with conductor length, this is problematic especially with the low voltage typically used in vehicles.

There are ways to achieve success using the right equipment, Victron is one manufacturer which specializes in automotive charging, simply connecting in parrallel and expecting a positive outcome will result in dissapointment.


Photon Sorcerer
Mar 20, 2021
Best to watch the voltage on the leisure battery. If the alternator holds 14.5+ volts on the battery terminals you can consider the battery fully charged in about 4 hours. If you only see 13.5 volts it could take a solid 36+ hours of continuous driving to fully charge the battery.

The voltage drop in the cable will reduce the amps flowing down to a trickle charge depending on size and length.


Photon Sorcerer
May 28, 2020
this method of charging was only likely to charge a lead acid battery to 80(ish)% due to the way an alternator works.
not quite true, there is a great deal of misleading information regarding RV service battery charging from the alternator and lack of clear understanding how a lead acid battery is correctly charged.

The van alternator is basically designed to service the base van loads, lights, engine electronics, air con fans, and so on. Additionally it will restore the small amount of power used to start the engine to the starter battery.

Most vans until the latest emissions standard, euro 6 (late 2015), had an alternator with a constant output voltage of 14 to 14.4 volts at the engine battery.
A VSR will switch the service batteries in parallel with the engine battery and allow charging. Due to the increasing internal resistance of the LA battery once it gets to around 70% to 80% it accepts less and less current before reaching full charge where the current falls to a low value, 0.5 amps per 100Ah of battery.
It will take a minimum of 5 hours for a 50% discharged battery to reach full charge with ideal conditions.
There are other factors than will compromise the charge process, volt drops in fuses, relays ,connections and cable will reduce the voltage that the service battery 'sees', resulting in a lower current during the bulk stage of charging.
In addition the majority of RVs use 3 way refrigerators, that operate on 12v when the engine is running, the often inadequate cable used, further increase the volt drop due to the current taken by the fridge.

The net effect is that although the alternator is delivering enough push. say 14.2 volts, the service battery 'sees' much less due to volt drops and a long charge time results. With most 'touring' drive times of 3 or 4 hours maximum the battery never gets to full charge.

To compare the MPPT charge profile of bulk, absorb and float, typical, 14.4 volts, absorb time 4 hours, float 13.8.
The the alternator charge profile of 14.2, drive time covers bulk and absorb, 'float ' is also 14.2.

The overall result of this type of system is that relying on a VSR alone will not fully charge the LA battery unless you have very long duration drive times and/or a low discharge of the service battery. With AGM batteries the situation is more serious with the low effective charge voltages and currents. This results in short battery life as the capacity gradually 'walks down'.

Vans from 2015 with start stop and smart alternators are unsuitable for VSR and need DC to DC chargers.



Photon Sorcerer
Jul 13, 2021
Don't forget modern smart alts that shut off entirely and definitely do not charge them to 100% by design.

I believe they're designed to keep a starting battery at a lower SOC of say 75 to 85% or something but don't quote me on the exact figure.

Dc to dc charger with the right features can detect this and disconnect the leisure battery when the alt output drops off so it doesn't start discharging back into your running vehicle.

That's what my camper does in my 2012 silverado. When the alt shuts off the bms reports about 7 amps being pulled out of it into the vehicle. This is solved by simply turning on my tow mode or headlights.


Solar Enthusiast
Jul 13, 2020
I didn't have any modern vehicles and I use an all in one camper (engine is in camper aka motorhome) and assuming I would do as I do which is to have a relatively robust 120VAC to 12V converter (aka 13.6-14.8 scaling charger) then what I think I would do is to find a relatively cheap 12VDC to 120VAC inverter and place it in a cool spot inside the truck or under the hood and run the 120VAC back to my camper and let the unit I'd already paid for in the towed camper function as the secondary B2B where the near the chassis battery cheap inverter would be my B2B sender unit.