Charging a battery bank with another battery bank, and waste not MODBUS etc

[jedon]

I have 2 Classic 150's hooked to 40 195W PV panels.
I have a Node-RED setup that receives data from the classics with MODBUS.
I previously had a bank of 16 GC2's which I now have in reserve.
I also have an older Xantrex SW+ 5548 as a reserve which I've hooked to the bank of GC2's
My newer active setup is a Conext 6048 with a 56Kw/h forklift battery.
We use a lot of power, it powers two residences etc, and even in the summer the generator sometimes kicks in around 6am.
I want to use the GC2 bank to charge the forklift battery when it gets low, and to charge the GC2 bank with the wasted solar from the classics when it goes into absorb or float.
I also have an IOTA 48V 1K charger. Right now what I've come up with is crude and wasteful, I hooked the IOTA up to the old 5548 which I keep on and I have it on a smart outlet scheduled to come on 5:30am to 7:30am ( I just hooked this up, not sure how that will go ).
Could I instead use say a DC SSR to feed the GC2 bank into the forklift bank?
And/or a DC SSR from the AUX on the Classics to charge the GC2 bank?
Can I get the Waste Not stats via MODBUS? I see where I get set the HIGH LOW points and I see things like DIVERSION HIGH PWM but not sure how to use that stuff.
What I would like to get set up is to have a full standby system plus use it as a reserve.
I'm a total nerd so go technical :-)
Thanks!
-Jedon

[ClassicCrazy]

PWM is useful for putting power into a resistance load - you can't use it with other things since it is switching on and off fast.
You could probably use Aux 1 in Waste Not to turn on a DC SSR relay that is rated to handle the power you want to switch.
Waste Not is designed to just take the extra power from your system while maintaining it in Absorb and Float. You would need a controller of some sort in front of the second battery though.  I don't know how it works with AUX1 as I use Waste not with Aux2 in PWM mode into a resistance water heater coil.
I suppose you could also use the Aux1 on Classic to switch on your battery charger. That way the second battery would have all available extra power but the charger would be in control to make sure it doesn't over charge.
You can use Node Red to do just about anything too as far as switching things on and off and writing the logic for that in Node Red.

Larry

[boB]

Nerds ?  We love nerds around here !

OK, so not exactly sure what you mean by "stats" in this regard.  There are the set-points for Waste-Not as well as the usual battery voltage and charge modes, etc that the WN tries to keep going, like, Absorb timers when it is voltage regulating.

There is also an AC diversion waste not and a DC mode for diversion waste not.  When in DC mode, it drops the PWMing frequency a lot so that the inductive resistors don't make things overshoot so much.  If you can get away with AC mode, that would be best as it is 500 Hz.  We did the DC mode mainly for our Clipper which has inductance problems if the frequency is not lowered.  The hysteresis is wider in DC mode though at the lower frequency (around 20 Hz) and so I don't really like it so much.

It has been a while since I have dived into the MODBUS regarding this so ask any questions you want to and we'll try to help out as much as we can.

boB, fellow nerd

[boB]

Oh yeah.  Crazy made me remember.  Aux 2 is the PWM'd port.  Never did quite get around to making Aux 1 work in the same manner as Aux 2 for diversion and such.

[jedon]

Thanks everybody!
Ideally I'd get a value over MODBUS that says there are 546 watts available and then have my system control what happens.
Lately I've been liking using ESP32 LoRA LCD boards for controlling things, so handy to have a screen, long range radio and lots of memory and CPU all on one $15 board.
Let me ask one question at a time.
So can I do this? Or is the micro charging issue I've read about where the battery bank doesn't like the pulsing?

[jedon]

I'm too full of questions!
Could I maybe input the PWM signal into my ESP32 and decipher the excess power amount from that signal? If I know the highest frequency and lowest, and I know the watts being output I should be able to calculate it. 
Can I just put a DC relay or SSR plus a diode between the two battery banks and let the GC2 bank charge the forklift battery directly?

[ClassicCrazy]

How much different voltage is the second battery than the battery pack the Classic is controlling ?

Larry

[boB]

How much different voltage is the second battery than the battery pack the Classic is controlling ?

Larry

That is a great question !!

If you have 2 large-ish batteries and a volt or more between them (or so) you might be talking hundreds of amps !

More info needed I guess.   
Amp-Hours and voltage etc of the banks

[jedon]

Forklift bank: 1182 ah voltage when I want to charge it a little, 48.7 average over 5 minutes, under 400-1000W load.
GC2 bank: 400 ah when new, if fully charged then 50.8 but maybe more like 50

[boB]

If the batteries are the same chemistry, you could simply just charge them in parallel and not worry about the SSR

[jedon]

It's my understanding that paralleling banks of such disparate storage capacity, condition, and charging voltage is not a good idea? I charge the forklift battery at 60V, the GC2's would boil at that voltage?

[ClassicCrazy]

It's my understanding that paralleling banks of such disparate storage capacity, condition, and charging voltage is not a good idea? I charge the forklift battery at 60V, the GC2's would boil at that voltage?

I used to have a camper and had some wires going from the front truck battery back to the solar setup I had in back. I don't remember if I had an isolater ( diode) in between.  But the back batteries never got that much current because the voltage differential between the front battery pack and the back one wasn't that different.  The truck battery was regulated to 14.4 or whatever they do. A solar panel is putting out a lot higher voltage so it can force more current into the battery .  So just having a direct line between the two batteries probably wouldn't over charge them. The thing you would want to make sure is that the batteries on the pv main pack went through absorb and into float before you connected them to the other battery pack.
If you want to really charge more amps from the first pack into the second pack then you probably need to rely on inverter to go to your AC charger . Of course there is inefficiency in that since you loose power at the classic, then loose power at the inverter, and loose some more power at the AC charger.
The more ideal approach would be to fill the first battery up and then switch some of the PV from first system to go to another charge controller on the second system. The thing is you need something that is capable of switching the PV voltage and current . There are some DC relays rated to 230v and 100 amps but the other solution is to use IGBT block transistors that are rated to handle higher DC voltages and currents. 
Maybe after you think about doing all that it might be easier to just put up some more PV for the second battery pack if possible since it would have it's own controller.

Larry

[qrper]

Want another crazy way of doing that?

How about a dc to dc buck/boost inverter and forgo the dc to ac inverter then back to dc.

Current limits would be an issue with most buck/boost inverters, but a think you maybe perhaps might wrap a power MOSFET around the output to increase current. Then again, I've seen some Chinese devices rated at 20A, so even if you had to parallel in a few, it might work.

Decades ago, while working with one (at the time) larger PV dealers here on the east coast, a customer had a similar problem. He had flooded cell lead-acid batteries, the 2 volt heavy as hell plastic cased monsters. He purchased an extra one for backup.
The trouble was keeping it at a float charge without running into the same problem of dc to ac then back to dc with an external charger to charge one 2 V cell. I build a DC/DC buck converter using a Texas Instrument IC (can't remember the part) and set it to 2.2 V and feed it off of the main battery bank of 48 V. Worked like a charm and as far as I know, the system is still up and running.

Anyway, just an idea

mike, wb8vge

[jedon]

Thanks everybody! You've given me a lot to think about and some things to try.