A few choices:

Are you only powering 24 volt items? Simply charge the batteries in series with a 24 volt charger.

If you need to power 12 and 24 volt loads simultaneously you'll need to charge the batteries individually. You can do this using two 12 volt chargers or use an automatic charging relay.

If this is on vehicle you can do this with 12v alternators but one alternator must be isolated ground. This is how the military CUCV trucks do it.

You cannot use a 2 bank charger as they have a common ground

Thanks TugboatEng

this will need to power both 12v and 24v loads. charging will be done via DCDC

would anyone be able to point me to an Automatic Charging Relay that could accomplish simultaneous 12v and 24v load and 12v charging from DCDC/ am getting easily lost in the internet, its not clear to me that the ones i see support what i'm looking for.

i've seen these tollbridges which looks like what i want, but it appears that they don't support simultaneous load and charge...uless i'm missunderstanding anything
https://drewcraft.com/product/trollbridge24-combin...

I'll have to look into the charging relays a bit more. I've never needed to buy one.

Can you give some more details about your application? Are you using an alternator at all? You mentioned automotive. What kind of currents do you expect from each load? Does the charger need to be able to maintain the load continuously?

@dmielke,
That's basically and DCDC converter that pumps up 12VDC to 24VDC. The device is basically a doubler. Seach the web using keywords "voltage doubler".

The Trollbridge (for fishing boat trolling motor batteries) doesn't step up voltage. It switches between charging the batteries in parallel and puts them in series for 24V operation.

"It works by automatically putting two 12 volt batteries in series when you need to run the trolling motor and in parallel for charging." from the website.

You can't simultaneously charge 2 x 12V batteries in series with a single 12V charger.

You can switch the charger between batteries as long as the charger output is floating or isolated from ground. Any control system you'd want to build with a 2-pole relay for each battery could do it. Basically, use a relay to disconnect both charger leads from one battery and another relay to connect both charger leads to the other battery. Then, switch back and forth between batteries.

The simplest is to get 2 chargers that have isolated outputs and use one charger for each battery.

Yes, ultimately the chargers have to support the load. They don't have to support the load all the time, but they have to charge the battery more than it's discharged and they have to do it quick enough that an intermittent heavy load doesn't over charge a battery.

Does anyone else remember the series-parallel switches on older heavy trucks?
Two batteries in parallel for 12 Volt loads and 12 Volt charging.
The switch momentarily re-connected the batteries in series for the 24 Volt starter.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

Does anyone else remember the series-parallel switches on older heavy trucks?
Two batteries in parallel for 12 Volt loads and 12 Volt charging.
The switch momentarily re-connected the batteries in series for the 24 Volt starter.

The Delco 11119845 series parallel 24Vcrank-12Vcharge solenoid

3 stage, two sets of NC contacts and one set of NO contacts, which is another style of wiring two 12V batteries in series for 24V starter cranking and 12V charging/run. The starter load is on the red cable on the right mislabeled "volt starter solenoid", the running loads and charging system would connect on the 12V(+).

Some 3 stage solenoid models (drawing in the post above), so equivalent models can be sourced.

Won't work for continuous series connection charging and use though.

LOL, there won't be any amount of useful charging if you charge two 12V batteries in series using a singe battery charger connected to the series string.

Amit Bajpayee is a nonsense advertising poster. It reflects poorly on the company he represents.

Ya, that kind of stupid is totally acceptable here now.

A single 24 volt charger works just fine to charge a pair of 12V batteries connected in a series string provided that the batteries are only powering 24v loads. Heck, each 12 volt battery is literally six 2.1 volt batteries connected in series string.

Every idiot should know that a 24V charger will charge 2 x 12V chargers in series.

The original question is the point being commented on. With a 12V charger, the batteries won't receive a useful charge.

Quote:

If you attempt to charge 24V series batteries with a 12V charger, the batteries may not receive a full charge

May not receive a full charge? No useful charging would happen.