24V alternator as input to MPPT

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24V alternator as input to MPPT

Postby bodgermatic » Tue Sep 02, 2014 12:17 am

Is there any reason why using a ~24V alternator as input to an MPPT solar charge controller to charge a 12V battery wouldn't work? The vehicle battery system is 24V, and I want to fit a 12V leisure battery - MPPT solar charge controllers seem to be cheaper than DC/DC 24->12V battery chargers.

Cheers,

Bodge.
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Re: 24V alternator as input to MPPT

Postby Gordon-Loomberah » Tue Sep 02, 2014 7:15 am

Welcome to the Energy Matters Forums Bodge :)

It works perfectly well, I've been doing it for years to charge a 24V battery. If it is a decent quality MPPT charge controller, and it doesn't have a 24V battery directly connected to it, then the alternator's output voltage can rise well above what it normally does in a vehicle, allowing significantly more output than the alternator is rated for.
For example, a 24V 30A truck alternator I used to use would put out 30A directly, but via my BlueSky SB50, it would go up to ~45A, as the SB50 would allow it to run at over 40V. If you are running it from a vehicle with a battery, it will function like an efficient DCtoDC converter.

However, good quality CCs are usually more expensive than converters, be careful with cheap MPPT CCs from ebay, often they are not really MPPT. How much charging current do you want?
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Re: 24V alternator as input to MPPT

Postby bodgermatic » Tue Sep 02, 2014 5:17 pm

Thanks for the reply Gordon :)

The alternator will be charging the vehicle starting battery, and the MPPT will be connected to the starting battery terminals via a split charge relay. I presume that the output of the alternator will be limited by it's on-board regulator. Is there any situation in which I could end up overloading the alternator or damaging the starter battery?

I've been using one of these: https://www.rolasolar.com.au/solar-char ... -2012.html For a couple of years now to charge a different 12V bank from a 24V nom. panel - it's definitely real MPPT (has a couple of hefty torodial inductors and is well heatsinked). Now a friend with a truck with a 24V starting system has asked me to investigate a separate leisure system on the cheap, so I'm looking into the alternatives. I think she'd be OK with 20A into the leisure system - it won't be a huge battery.

Cheers,

Bodge.
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Re: 24V alternator as input to MPPT

Postby Gordon-Loomberah » Tue Sep 02, 2014 6:01 pm

bodgermatic wrote: Is there any situation in which I could end up overloading the alternator or damaging the starter battery?


The alternator's regulator will keep the voltage correct, or else be at full current output on the way up to charging voltage, when the battery is fairly flat. So long as the alternator is of decent quality and not over-rated, it should be fine under all circumstances. Adding and extra 10A load (~20A into the 12V battery) would have a similar effect to adding a couple of 100W driving lights- no big deal.
As far as damaging the starter battery goes- remember that it is a starting battery, not a deep cycle battery. That means it is designed to deliver very high current for short periods, and be immediately recharged. Leaving it running loads via the MPPT charger and 12V battery for long periods so that it gets to a high depth of discharge will damage it, and kill it pretty quickly if done very often.

I cant get any rolasolar web page to load.
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Re: 24V alternator as input to MPPT

Postby bodgermatic » Tue Sep 02, 2014 6:11 pm

Gordon-Loomberah wrote:I cant get any rolasolar web page to load.


Me neither, now. Was fine when I linked to it. Weird :|

The charge controller will be isolated from the starting battery with a split-charge relay, activated only when the alternator is running. So that will avoid loading the starter when it's not being charged.
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