All books tell us not to use 2 different batteries in parallel, well, see below.
We are charging 2 different lead acid battery banks (even different brands) with the same charger/inverters for 12 years now.
E.G. One is 1820Ah (small one) and the other 2383Ah (big one).
The big one (master) is connected directly to the busbar, the small one passes first 2 pieces 660 Amp mosfets (connected opposite) to charge and to invert, mounted on a cooling device.
These mosfets are regulated with a PID regulator which takes the signals (voltages) from the individual shunts (BMVs) through 2 isolators (50mV to 10V).
The mosfets are then controlled through 2 isolators (0-10V) as well.
The switching between one and the other mosfet when going from charging to inverting (and vise versa) is not fast, about 1 second, as it has not to be fast since the big battery can handle all load on itself without dropping the voltage.
So, as can be seen, these batteries are always connected with each other but separately charged and discharged, about 1 to 2, e.g. the small one 25 Amp, the big one 50 Amp.
Well, see it as a BMS from a Lithium battery, but then better, as a BMS is mostly NOT controlling the discharge of a battery.
Due to the mosfets, which have a very low internal resistance, one battery will not “charge” the other.
My question, are there any others who have 2 different batteries connected in a similar way?
Question: Stupid question but just to be clear… Mosfets connected opposite in series or in parallel ?
Why this question? Because one could easily forget about the reverse diode inside mosfets…
And if they are in parallel and the delta V between batteries is bigger than the diode drop voltage, then the current will flow anyway, no mater the state of the mosfets…
We are starting from the idea that both batteries have the same voltage and chemistry.
Then it really has to do with how the “switches” (read mosfets ) are operated.
What I mean is that if you have the big battery charged and small battery discharged, if you open the “direction” from big to small (charging sense), the small one will charge.
And vice-versa, if small is charged and big is discharged, if you open the “direction” from small to big (inverting sense), the big one will charge.
All above because the internal resistances.
But if both have about the same SOC, meaning about the same voltage, then yes, they will not “dump” from one to another.
Anyway, you will always have a small voltage difference between batteries, because of the reverse diode of one or the other back-to-back mosfets.
As for not using 2 different batteries in parallel… well… a lot of battery boosters/starters are sold and used in automotive industry and no one complains about putting - ok, for small period of time - two types of batteries in parallel…
More of that, think about batteries that have the cells inside in a series/parallel configuration !!
No BMS there and, at a cell level, you have 2 or more “batteries” in parallel.
I may be condemned for the following idea, but sometimes, in my opinion, the recommendation are there for people that skipped physics and electricity classes at school… And for the other side for avoiding responsibility for that.
Indeed, parallel, minimum delta voltage of these MOSFETs are about (temperature) 0.16-0.24V at high amps, the diodes 0.5-0.6V.
The 2 batteries never become discharged or charged above delta 0.08V in between, normally zero.
That is all controlled by the PID regulator.
I know, there are companies in the Telecom centres who do the same, but, as usual, everybody keeps that secret for themselves…
You are absolutely on my side as I understood.
Shall I publish the items I used…since we can put many businesses out of there comfort zone with that?
Civil customers are put already 85-odd years on the wrong foot, E.G. submarines, these had this already in WW2, not with MOSFET but with these glass tube controllers…
I’ve seen 40-0dd year old lead acids from these subs on the scrapyard, still working perfect, ok, less potent of course, and these are very big…
Actually, I use a load controller from a vessel to do this, and 1980ties LCM card from LIPS (now Wartsila), in India widely available since there these vessels are dismantled…some in Turkey…
I even have several more at home, just in case one fails…my job…they never failed, ever, and I’m doing this already 30-odd years…
) are operated.