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Our garage has no mains connection, but it does have a 12V leisure battery charged from a solar panel, connected to an inverter to produce 240V AC. That mainly gets used for charging a mobility scooter that's kept in the garage overnight, and the mobility scooter runs off a 24V battery.

The current setup – converting the nominal 12V DC to 240V AC then back to (approximately) 24V DC to charge the scooter battery – seems grossly inefficient to me. It seems like it should be possible to set up a charging system that converts from 12V DC straight to 24V DC. But I'm utterly failing to find a sensible way to achieve that. What would I need to charge the 24V battery from the 12V one without converting to AC as an intermediate step? Or is it just A Bad Idea for some reason I haven't spotted?

Ideally I'd like the 24V system to be some sort of intelligent charging system, i.e. adjusting the charging voltage and current according to the battery voltage. I'd also vastly prefer to not need to rewire the scooter batteries from the 24V series layout to a 12V parallel layout.

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  • Just go 24v all the way through - panels to charge controller and then control charge to the batteries. – Solar Mike Apr 1 '20 at 12:20
  • @SolarMike that's definitely an option, but I'd prefer to avoid having to replace all the panels and control circuitry we already have… – me_and Apr 1 '20 at 12:23
  • Why would you need to replace the panels? put 2 in series then 12V * 2 = 24V... – Solar Mike Apr 1 '20 at 12:26
  • Any chance the 24V battery has a jumper so you could remove the jumper, charge both 12-V sections in parallel, then put the jumper back in to make it 24 V again? – Carl Witthoft Apr 1 '20 at 19:19
  • use a GAMA battery post disconnect switch between + and - where batteries are connected in series; wire up one 12vdc smart charger connecting in parallel to both + terminals and both - terminals. If you operate smart charger it will simply see a short connection and do nothing until you turn the disconnect unhooking the series connection between batteries; smart charger than charges two 12vdc batteries in parallel – ron Apr 1 '20 at 23:58
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It seems like it should be possible to set up a charging system that converts from 12V DC straight to 24V DC.

yes, "possible". You would do it with a DC-DC boost converter. you can look up types of dc converters on the web. At this level you basically take 12v @ 10 amps and turn it into 24v @ 5 amps, then you would just need some higher vdc than your battery to make current flow into the battery to effectively charge it (until you overcharge, cook, and ruin the battery).

Ideally I'd like the 24V system to be some sort of intelligent charging system

with that you are likely going to look for a smart charger sold that will output 24vdc. The typical smart chargers I think everyone is aware of are for 12vdc. And for 12v it is not 12.0 vdc that the typical lead acid battery is, at 100% charge is about 12.65vdc and varies with temperature. The battery at 12.20 volts is around 50% discharged, and at 12.0 volts is actually dead (not enough power left in it to be useful). That would correlate with a 24vdc battery however the math goes, something like 2.1 volts per cell in the battery, and some differences between lead-acid vs AGM vs gel battery types. The "smart" charger usually takes care of all that.

converting the nominal 12V DC to 240V AC then back to (approximately) 24V DC to charge the scooter battery – seems grossly inefficient to me

yes from a strictly power usage perspective. There will be however much inefficiency of the inverter from 12vdc to 120vac, and however from 120vdc to 24vdc smart charging. But in terms of cost and availability of equipment to accomplish this based on having only solar panel output, it is not "grossly" inefficient. And if you did the math for how much KWHr's are used in charging vs what is wasted from inefficiency it would be pennies on your power bill.

Any smart charger you would be buying is going to plug into 120vac. I seriously doubt someone is making a smart charger that runs off 12vdc (solar panel output?) then smart charges a 24vdc battery. If u asked this in the electrical engineering section someone would probably tell you what IC's to solder together to accomplish a simplistic regulated battery charging device.

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The current setup – converting the nominal 12V DC to 240V AC then back to (approximately) 24V DC to charge the scooter battery – seems grossly inefficient to me

It is. It's intolerable. The inverter has "standby" draw simply for being spun up, which will bottom out your battery, which is damaging to lead-acids.

Cannot be done sensibly.

It seems like it should be possible to set up a charging system that converts from 12V DC straight to 24V DC. But I'm utterly failing to find a sensible way to achieve that.

Because it cannot be done sensibly. Battery charging is an extremely complex science. Proper charge management is essential or lead-acid batteries will be destroyed. Nickel-cadmium is more resilient, but still, you can't just slam a charge onto it higgledy-piggledy. Lithium batteries are the worst of all; mismanagement of their charging will lead to explosion and fire.

Battery charging must be left to the professionals, or to be more precise, to professionally designed chargers rated for the equipment. If this equipment has an external battery charger, with the smarts in the external charger, it's simply a matter of going back to the manufacturer and saying "Yo, I need a car charger for this thing, here's my VISA card". And they say "okeydokey" and ship it off to you.

Or it may be possible to find competent COTS 3-stage battery chargers able to input 12V-ish. Your 12V system is, after all, quite similar to a car's electrical system. Charging a mobility scooter from a car is a common enough application.

Then you wire a "cigarette lighter" style socket raw/hot into your battery, and Bob's your uncle.

Or use DUMP.

If the scooter does not need to be charged on-demand, and your solar charge controller has a set of DUMP terminals, it can be feed off DUMP. DUMP is energized when the controller has excess power it can't use (because the battery is full and charge energy exceeds all available loads). So the scooter charges only after the battery is fully charged, using only solar power that would otherwise be thrown away.

For that matter, the inverter could be placed on DUMP, and it would only create inverted AC with excess power.

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    Well, it turns out DC-DC charging of that nature is common enough that DC-DC converting smart battery chargers exist! In particular, the Victron Orion-Tr Smart line comes in a 12/24-15 model that takes 12V in and charges a 24V battery from it, although there may be others on the market as well – ThreePhaseEel Apr 1 '20 at 23:29
  • @Harper Everything you've said is stuff I already know. A multi-stage 24V charger that takes a 12V input is exactly what I'm asking for. – me_and Apr 3 '20 at 9:01
  • @ThreePhaseEel perfect, that's exactly the information I was looking for. I'll go look at those chargers in more detail now. – me_and Apr 3 '20 at 9:01

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