# Charging 24V Battery with 12V Alternator and Isolator

Hey guys, I got a new trolling motor for my boat but it is 24 volts so I got 2 deep cycle batteries and I will be linking them in a series circuit. For charging the batteries I have a float charger for 24v but power is not always available which brings me to my diagram: Charging batteries linked in a circuit with a 12 volt source. I am not sure if this will work, I am open for other solutions if this is not possible.

Edit:I should mention the isolator keeps all the connections separate only to supply a one way feed of current not connecting the 2 batteries together

• Notice that when you close your isolator you have a dead short on the battery on the left. BANG! Aug 25 '16 at 22:25
• Agree, your diagram will not work as planned even with an isolator. Aug 25 '16 at 23:08
• @Pᴀᴜʟsᴛᴇʀ2 Would you have any other solutions to make this work? Aug 25 '16 at 23:17
• Assuming you are not using the load while maintaining a charge current, wire it up with a couple of disconnect switches so you can charge them in parallel. Apr 7 '17 at 19:39

No, your isolator circcuit won't work.

I think the only safe way to charge the batteries is to connect them in parallel.

The 12->24 volt converter suggested by @spicetraders won't work, because you will need about 28 volts to charge a "24 Volt" battery (or two, 12 volt, in series.).

The description on that eBay page mentions an Orion 12/27,6-12 which is apparently designed to charge a 24 volt battery from a 12 volt source.

• That is correct the charger mentioned is a better item, unfortunately could not find one listed only mentioned. But I have used the converter I posted in a solar system it can make near 28volts out. Aug 26 '16 at 0:21

You can us a siilar arrangement if, instead of an "isolator", you use a DPDT (double pole double throw) switch that transfers both leads of the alternator from one battery to the other. This allows the alternator to change one battery at a time.
While "doable" it's not a good idea s you need to make various compromises or take certain real world actions to treat the battery well. eg the changing should be moderately frequent to keep the batteries balanced, charging while trolling adds extra issues etc.

A 12V to 24V-proper-lead-acid-deep-cycle-battery-charger would be more satisfactory. Required Vout varies and a purpose designed unit will deal with this. Having enough voltage out is necessary but not sufficient to keep your relatively expensive batteries in good condition.

you can charge 2 12volt batteries at the same time that are in series using only 12v... the diode has to go on the + and - with each individual wire from battery on either side of the diode

crappy picture. but here it is.

I have not tested this. it's the only way i can figure it would work and I can't disprove it.

• This is a very intriguing answer and makes sense. I'd think your diode would have to be a monster, though. Something big enough to handle the amperage which a 12v alternator could produce (some are in excess of 100A). Apr 6 '17 at 20:39
• sorry, but no way this works. Your 12v in is directly in parallel with the 24 out. Do a better diagram if you aren't convinced. Apr 7 '17 at 19:34
• it works.... and my diodes are alldatasheet.com/datasheet-pdf/pdf/127261/SEMIKRON/… rated for 22.9 amps at 3200v so about a 7kw diode Apr 16 '17 at 12:34
• and as for agentp that doesn't matter. due to the fact that the jumpered terminal on the negative has a diode the potential can't come up higher than 12v unless you are connected to the opposite negative you're not going to be touching your 24v device to ground or whatever you need the 24 volt for in the first place... flow is negative to positive.. Apr 16 '17 at 12:36
• the problem has never been with the 24 volts.... its the fact you can't charge with 12 due to a direct contact with each batteries own terminals Apr 16 '17 at 12:42

Sorry, that will not work.

The negative side of the right hand battery will be at about 0.5 volts due to the diode. So the positive side of the right hand battery will be at about 0.5 + 13 from charger = 13.5 But the negative of the second battery is also connected to this so it too will be 13.5. The positive input for the left hand battery is also about 13 volts so there is no voltage across the left hand battery and it will not get charged.

You can purchase automatic series parallel switches to do this job. See www.trollbridge.info

– Mast
Nov 16 '20 at 18:49

This is kind of one for both here and hardware communities as a good electronic person would point you toward a DC to DC converter with isolation diodes. 8amp DC 12v to 24v

DC to DC converters come in many "flavors", with up converter from 12v to 24v being very common for use in solar power situations. The link is a 8amp version, but they can be found in much higher currents then 8amp. To make this a 24 charger you need. Add back flow blocking diodes for isolation. and a charge regulator.

Well, the best option is, of course, to just buy a 24-volt alternator.

Most (car) alternators produce AC that goes through a rectifier and a regulator. If this is the case, you could use a step-up transformer with 1:2 ratio, but it has to be designed for this kind of application as changing RPM changes the alternator output frequency and consequently would change the output voltage of a transformer. If you can find such a transformer, you will only need the rectifier/regulator box from a 24V alternator.

DC-DC converter is a good option, but again, it has to be designed as a lead-acid battery charger, not just a power source. As people mentioned, you'll need about 28 volts at the output to charge 24V system (a battery is made of cells, lead-acid cell is described here: https://en.wikipedia.org/wiki/Lead%E2%80%93acid_battery#Voltages_for_common_usages).

Another possibility (most feasible) is to buy a couple-hundred watt invertor and a 24V standalone charger (designed to work with 100-240VAC depending on your location).

I found a simple affordable solution to my issue, if anybody is in the same boat there is a product called trollbridge which combines the batteries for trolling and separates for charging. It auto switches to 24V when a draw is detected and will not allow them to charge when in 24V mode. it's not recommended but you can draw from the bateries in 12V separately when it's in 24V so my radio still works when trolling. I ordered from the website below and it came fast installed and working flawlessly, they also have other products for battery soloutions. Hope this is able to help you out too.

http://www.yandina.com

Two options I see that are relatively safe/easy:

I would look into hobby chargers like the iCharger series. They have built-in DC-DC converters and can be powered by 12v (do not use the cigarette lighter jack, those are usually only fused at 10-15a) -- add a fuse off the battery at something like 20% above the charger watt rating and set the charger voltage/rates according to the 24v battery chemistry. These will monitor the voltage/current of the destination appropriately.

If you have an AC charger already, price out a 12v inverter -- preferably one with an adjustable/appropriate low voltage cutoff (or wire one in) to kill it if your engine battery voltage gets too low. Give yourself some headroom compared to the rating and you will need pure/true sine wave for most charging applications I believe.

I tried a buck booster to 29v but found one battery would start taking most the charge. Contemplating,reg-micro connected across the 24v to switch a NPN fast for the Alt / solar input - {elec cap before NPN mosfet to store charge in off times} into a hf transformer {SMP - rewired} to have two 14.8v out put coils {bridge rectified and caped / elec} into each battery, but to then have an NPN on each batt with its own reg and micro controller {on that battery} set up to read that batteries voltage and cut-off - cutting off the NPN. Blah. Its quite complicated and depending on your situation - time money thing to buying a 24 volt batt & charger. The other option was to have PNP and NPN Mosfets for each batt set up to alternate battery on switching, but theres problems with having the micros connected {earth problems} - so I'll try transformer. Ive just had a set up that switched npn and pnp alternatively with small delay between switching connected to the junction 'to avoid collision' {battery series connection} with Alt / solar feed connected pos and neg to free battery terminals - but that was a disaster. I dont see how Bruce Harveys circuit can work. The negative diode is wrong way around so will do nothing, if the other way around the alternator will short via top diode.

Has this been resolved ? To save a lot of messing about trying to get 24v from 12v to charge them why not just go old school and use a multi pole relay (with a 12v coil)to disconnect the bridge and connect the batteries in parallel to the charging system.