DC to DC charger

5082451819

Cab I use an A/B switch after a DC to DC charger to alternate the Charge between 2 separate battery banks , one lifepo  with an appropriate charge controller and one LA .  The battery banks each have their own wiring system and never integrate, except at ground. 

BB.

Most hand operated A/B switches should only be switched with "load" (and charging) current off (prevent DC Arcing).

Also note there are several types of A/B Transfer switches... There is A or B or Off switches. And A/A+B/B/OFF switches... Just make sure you get the one you need (i.e., do you want the ability to connect the two banks in parallel (A+B) or never (A or B or Off). 

Generally, you want to connect the charger to the A or B bank, then power on the charger (so the charger can sense proper bank voltage before starting).

Of course, you will probably need to set the charging parameters based on which bank you are charging (to a degree, this will also depend on the BMS for the Lithium bank--Some BMS+Lithium batteries try their best to "look like" Lead Acid batteries during charging).

Note that, generally, lead acid batteries need a temperature sensor to adjust the charging voltage based on bank temperature (hotter bank, lower charging voltage). Lithium batteries generally have zero temperature offset for charging voltage...

And here is where I try to avoid. I don't like to set up a system that can damage (batteries in this case) my system if I forget to change some parameters as I switch between A and B banks.

It will probably happen sometime/somehow... You are heading out of the place for an urgent drive somewhere--Or you tell your spouse to power down the charger, switch A to B, and power up the charger and reprogram for B bank... And they don't do it correctly.

FLA batteries are pretty forgiving. AGM don't like being over charged (venting issues), and Lithium Ion can self distruct if over or under voltage (BMS should help limit voltage/temperature/current before damage occurs).

Personally, I would suggest two different chargers (one for A bank and another for B bank) that are permanently programmed and connected as appropriate. If you have energy limits (say genset can only charge one bank at a time)--Then you may want to an A/B transfer switch on the AC going to the chargers...

In the end, make sure you understand the requirements for your batteries, chargers, and how they need to be connected/switched (under load or not, XX VDC side or XXX VAC switching, etc.). And to not setup situations where simple mistakes can cause problems.

-Bill

NANOcontrol

I would provide a disconnect to remove PV voltage before switching batteries. Connecting two banks together of different chemistry for even a short time can cause extremely high currents.  At my camp I have a li in parallel with a lead acid. The LI is at the house and LA at the power shed is some distance away. That prevents high currents. A diode voltage drop makes the two batteries in parallel on same charger compatible enough. A manual topping of Li is done occasionally.   

5082451819

Thanks Folks, great points.