12/24 volt system

The design
Two 12v batteries in series a and b. ( - bat a wired to + bat b )
+ bat a and - bat b make 24v for windlass.
+bat a and - bat a make 12v for boat systems
+ bat b and - bat b make 12v for boat system
+ a and + b go to positive buss bar
- a and - b go to negative buss bar
Charging goes to pos. and neg. buss bars
Comments ??
Thanks. Nick

Not ideal for the health and longevity of your batteries.

Consider using a 24V to 12V converter instead.

However, charging the batteries wired in Series for 24V independently at 12V using a dual bank charger or individual chargers has some advantages as the batteries age and drift in performance.

No.

I have a 24v system. I have about 6-8 solid state dc adaptors that take the voltage down to 12v..N2k, Furuno radio, some various other stuff I could not get in 24v

Charging and discharging the 24 system unevenly is a major no-no. The batteries charge or discharge unevenly and life is aggressively shorter. Like, as in 6 months would be good.

If you only want to run the windlass at 24v, you can get a step up transformer, but it'll need to be rated for the amount of current.

here is a 20amp

Victron Energy Orion IP20 12/24-Volt 20 amp DC-DC Converter Non-Isolated, High Power https://www.amazon.com/dp/B01BF5BU20/ref=cm_sw_r_cp_api_i_PTY5T1SFH9XM03Z5E6F0?psc=1

STOP WHAT YOU ARE DOING IMMEDIATELY!!!

1.) Battery A should not be used as a 12VDC system since there would be two wires serving as DC ground, which can lead to confusion, shorting a battery, and possible fire.

2.) Unless there is a device to switch the batteries between series during use and parallel during charging; as described both batteries will short and cause a fire.

Please draw the proposed circuit to clarify intentions, when in use and when charging.

Could you use a series/parallel switch? Some trucks used this to drive a 24v starter while maintaining a 12v system otherwise. Not seen very much anymore but still available.

The problem with what the OP proposed is that he is trying to use his system as both 24v and 12V, and on top of that wanted to use each 12V independently to power 12V loads.

When batteries are wired in Series, their voltage and capacity should be equal, otherwise the higher will move energy into the lower to charge it.

So if you end up drawing different amounts of power from each 12V independently, they will be constantly moving energy between them.

To make matters worse, one battery might end up doing the majority of the work which then affects its performance with how quickly it can recharge and reach full capacity, which causes the two batteries to be out of sync.

Then when you apply either a 24V charge to the Series bank, or a 12V charge to the batteries in Parallel from the Bus bar, one battery will reach capacity before the other, maybe even exceeding it, while the other is still working slowly to recharge, and now you are overcharging your good battery while the other tries to catch up.

And the cycle just continues until you kill a cell, then eventually that bad battery kills the other as you experience diminished capacity and start to draw below the healthy discharge floor because one battery has now been asked to do all the work, etc..

The simplest thing to do is either use a step up converter (12V to 24V) to power the 24V load as Fallguy suggested, or if this is a solar install where a higher bank voltage of 24V is advantageous, you can use a step down converter (24V to 12V) to power 12V house loads.

Remember that any time you wire batteries in Series or Parallel, you want to keep the wires equal in length and gauge.

If you have a lot of 12V house loads, I would wire up a step down converter to a Bus bar and tie all 12V loads to that. Then wire your 24V load directly to the Series bank making sure proper fusing and switching is used.

Victron and others make step down converters that can deliver 50+ Amps.

Also as I stated, using a multi bank charger to independent charge each 12V battery in a Series string is beneficial, versus supplying a charge in Parallel.

NOCO makes some great Marine multi-bank chargers for a reasonable cost.

I would look into products from Yandina they are very well made and have many solutions to this type of setup. Also agree with NOCO chargers best bang for the buck out there.

Like kayakmarathon says: you're making a full shortcut if you would wire it as you describe. The other contributors give goood advice how to tackle this.