Midnite Kid using load to charge a secondary battery bank

[241comp]

I have a Midnite Kid with a 1200W solar panel array and primary 48v battery bank (4x 12v AGM Group 4D) in an off grid cabin.  The Kid maintains the bank while I am away so it is charged when I arrive.  I have a secondary 48v battery bank (4x 12v AGM Group 31) that I use during extended stays.  I don't want to combine the banks for charge/maintenance when I am away because they are different batteries with different capacities and charge specs.

Can I use the load function to charge/maintain a secondary smaller 48V battery bank when the primary bank reaches full charge (Float ON or Diversion)?  Reading the manual it appears the load function should output 48v at 30a.  Is the load output regulated to 48v or does it match the battery bank actual voltage (about 54v)?  Can I get two 24v PWM chargers, one connected to batteries 1&2 and the second to batteries 3&4 and power them from the load output (in parallel - 48v in / 24v out)?  Is the load output voltage high enough (ie 54v) that I could power a 48v PWM charger?

[boB]

The Kid's Load output is not a voltage "source" but instead a transistor (MOSFET) that can pull a load's negative line to battery negative.

Open drain as they say.  Kind of like a relay contact but good for only one polarity or direction.

So, it will not work as a battery charger by itself.  It could possibly control a relay coil so that its contact might turn on and off some kind of power supply but that might be about it.

Since I haven't thought too much about this, someone here on the forum may think of some great way to do what you want to do though !

Let's see what happens.  There are some smart people on here !

boB

[241comp]

Thanks for the reply.  If I understand the manual correctly, the positive "load" terminal is direct to the battery bank and the negative "load" terminal is a switched /  connection to the battery bank, is that a correct understanding?   This would mean that my primary bank would essentially be made available as a power source on the load terminals (max 30a) at about 54v when in Float.  I could then drive a small dc-dc battery charger of some sort to use the battery bank to top up / maintain my secondary bank.  In this case, I'm proposing one or more small pwm solar chargers as my dc-dc chargers (they are dirt cheap compared to what I'm seeing for non-solar dc-dc chargers).

[ClassicCrazy]

Thanks for the reply.  If I understand the manual correctly, the positive "load" terminal is direct to the battery bank and the negative "load" terminal is a switched /  connection to the battery bank, is that a correct understanding?   This would mean that my primary bank would essentially be made available as a power source on the load terminals (max 30a) at about 54v when in Float.  I could then drive a small dc-dc battery charger of some sort to use the battery bank to top up / maintain my secondary bank.  In this case, I'm proposing one or more small pwm solar chargers as my dc-dc chargers (they are dirt cheap compared to what I'm seeing for non-solar dc-dc chargers).

If you just want to maintain the second battery pack why not tie it together with the main pack and just use a relay between them which you can control with the load terminal. There shouldn't be much current flow between them if the voltages between the two battery packs are close.
Larry

[241comp]

Great question, and that might be a reasonable approach.  I'm just not sure as my batteries have fairly drastically different charge specs and float voltages due to different batteries and ages. Additionally, both banks may be starting out drained from use.  Im concerned that attaching my drained secondary bank directly to the primary bank might drop the charge out of float, which will then disconnect the load terminals and it will flip back to float again, in a cycle.  Do you think that is a risk?

[ClassicCrazy]

yeah  - it is different if you are trying to charge a low battery pack.
When you said maintain I thought you meant to just keep a small charge in an already charged battery.
Not sure what you mean by drained battery pack . But with lead acid the deeper you cycle it the less longevity the battery will have.
There are graphs that show this on battery specs.
If you want a battery to last 10 years or more you only take 20% out of it.
Larry

[241comp]

Thanks for the advice.  I have my inverter configured to automatically shut off at 80% depth of discharge and I rarely allow it to go below 50%.  My primary battery bank is made of industrial deep cycle batteries rated for an unbelievable 300 cycles at 100% DoD (these: https://discoverbattery.com/products/search/EV4DA-A).

That said, this is where my secondary bank comes in.  I use it as emergency capacity when my primary bank is low and solar can't keep up to extend the life of the primary bank.  This bank never goes below 50% state of charge.  When I leave my cabin, either one or both banks may be as low as 50% state of charge.  If lower, I run my generator for a while to charge them up.  If above 50% state of charge I just let the solar charger fill the primary back up while I'm gone.  It always reaches 100% by the time I return for my next visit, usually charging in less than a week.  I want to use the excess solar to also charge up the secondary bank.

[ClassicCrazy]

I just read up on those batteries and the company. Never heard of them before.
I see they have a line of solar batteries too.
The last batteries I got are called lead crystal type . They are also supposed to be able to go 100% discharge with no sulfation.
So far they have been working okay for me.
I would like to get lithium but need to make a warm place for them which right now the batteries are out in the cold .
Sort of in the cold - this fall I built a crude insulated box for them made out of foam board left overs.
I have a space heater in there so I can heat up the batteries which is nice. If I had insulation under them that would have been keeping them warmer over night.
But big job and a summer job to pick up all the cells and put on something insulated.
Larry