Charging Sun Xtenders at Low Temps - Help Please!

[Boompy]

Hi guys,

I'd really appreciate some feedback from people who have experience with low temperature (-25degC / -13F) battery charging.

My scenario is a little unique. I am looking to charge a 400Ah 24V Concorde Sun Xtender bank. In a worst case scenario, the batteries might be at -13F (but SOC would never be less than around 80%).

I have both a DC genset easily capable of 100-120A charge current in conjunction with a 500W PV array.

This is all great, only the DC genset has no temp compensation. The Midnite solar charger controller does though. Generally we would discharge our batteries to an approx 80% SOC - we have a shunt that measures nett current consumption and we can compensate for temp and charge inefficiency in this calculation.

My problem is this, the DC genset basically has a fixed terminal charge voltage of 28.8V. In a more conventional scenario I would run the genset for the purposes of bulk charging and then let the PV charge controller (which has temp comp) take care of the absorption and float. The genset is just there to deliver raw bulk current.

My worry is that the fixed 28.8V genset ceiling will prevent me from charging my batteries from a depth of say 80% SOC. Reading the Concorde battery manual tells me I should be charging at around 31.5+V.

So there's my question. If I have 100A + generator, with a terminal voltage / set point of 28.8V, will my batteries charge? Will it just take forever to get to close to absorption? What if I limit my SOC to a minimum of say 90% and use the genset from there?

Please help

[bcroe]

If the genset is producing exactly 28.8 VDC, it must have a voltage regulator. Perhaps the regulator
can be adjusted. Bruce Roe

[Boompy]

The genset has a setpoint of 28.8V and gradually works its way up to this target, at which point it holds constant. Granted, I can have this setpoint changed to a higher value, however this would be problematic in higher temperature environments (voltage too high).

The system is unmanned and all voltage presets are programmed at factory, it is not field configurable.

[Sunking]

The genset has a setpoint of 28.8V and gradually works its way up to this target, at which point it holds constant. Granted, I can have this setpoint changed to a higher value, however this would be problematic in higher temperature environments (voltage too high).

The system is unmanned and all voltage presets are programmed at factory, it is not field configurable.

Can you afford another Charge Controller? If so have the genny voltage raised to say 34 volts, and use second charge controller to knock the voltage down. Just one catch, the CC must be MPPT.

[Boompy]

Ok wow, now you've really piqued my interest. So you are suggesting I use say another Midnite Classic and feed the PV circuit with 34V from the DC genset and use this device to manage the temp compensated charge. That's an interesting approach and something I'd be keen to attempt. Thanks.

Out of curiosity what is your take on my previously suggested approach? What is likely to happen when charging at my 28.8V fixed voltage? Will I simply struggle to deliver the intended charge current and stagnate? I had a feeling the batteries would naturally rise in temp and eventually become more conducive to accepting charge current. I had this idea that I'd simply be running the genset for extraneous periods until things warmed up a little.

But gee thanks for the tip, that's a really neat idea and something I'd be more than prepared to work with. Awesome!

[Sunking]

Ok wow, now you've really piqued my interest. So you are suggesting I use say another Midnite Classic and feed the PV circuit with 34V from the DC genset and use this device to manage the temp compensated charge. That's an interesting approach and something I'd be keen to attempt. Thanks.

Exactly and you are welcomed.

Out of curiosity what is your take on my previously suggested approach? What is likely to happen when charging at my 28.8V fixed voltage? Will I simply struggle to deliver the intended charge current and stagnate? I had a feeling the batteries would naturally rise in temp and eventually become more conducive to accepting charge current. I had this idea that I'd simply be running the genset for extraneous periods until things warmed up a little.

OK had to read your opening statement to get your drift. Bottom line is the stiffest source supplies the power. In other words the source at the highest state of energy. So now you need to understand how a battery charges up. Keeping it simple when battery voltage and the charge source are equal, current stops because there must be a potential difference. With me so far?

So let's say you set the Genny CC to 31 volts and the panel to 33 volts with both CC's to the same Temp Comp Curve. The Genny will do most of the work, until the Panel energy state becomes higher than the Genny.

Another way is to use MS controller logic controls, and use an external Logic Controller. Let the Genny charge the batteries up say before sunrise. When the Genny CC completes the charge cycle, the logic controller locks the Genny out, and enables the Panel Controller during the day.

[Boompy]

Ok, I hadn't considered the PV potential competing with the genny.

I was more concerning myself with the batteries inability to accept charge current on account of it's low temp and subsequent insufficient genny charge voltage of 28.8V.

So basically my setpoint for the genny CC can still sit below the PV CC setpoint and do its job. As far as the supply side is concerned, I'd just run the genny MPPT charger in parallel with the PV CC with a supply voltage of say 34V.

I'd just need to confirm whether a Midnite Classic would be capable of handling around 90A charge current on its PV input - I know its rated capacity is 96A, but on the PV input???

If somebody sold a battery charger that connected directly to a DC genset, I'd buy it. But I get what you're saying. I could use the MPPT CC for this purpose.

This is a really interesting thread, thanks for your input

[littleharbor]

Midnite Solar Classic max output varies with battery bank voltage and input voltage. Maximum 94 amps at 24 volt nominal.
If this can be opened look on page 15 at the graph for Classic 150. It does appear the lower input voltage results in higher output amps. Thing is the lowest voltage on the graph is 70 volts.
Midnite Solar Classis.pdf graph

[Mike90250]

I'd look into being able to adjust your DC genset regulator to get the needed winter voltage, could be as simple as a couple diodes stacked on the ground reference to raise it up a bit. As a shorting switch for summer.

I'm wary about using a charge controller (even a top of the line MPPT controller) to manage a non-solar charging source. If the MPPT has a wind or hydro mode, those resemble a wound alternator, and not PV panels. The loading algorithm for PV may make the genset unhappy.