Using charge controller and generator to charge batteries in unision.

Hairfarm · May 2015

Greetings,

As someone new to the game of off-grid solar I'm not sure what the best way is to charge my battery bank using a charge controller and generator, both at the same time.

I have a Flexmax 80 CC (Outback) and a 2k inverter-charger (Prosine 2.0). The inverter-charger has a 100A multi-stage charger built into it. The Prosine 2.0 will take shore power (in my case a 5k generator) and charge my batteries with it (12v).

On the occasion that I need supplemental charging from my generator, what is the best way to do this? In other words if my panels can't quite get my batteries fully charged (too many cloudy days, too much load on a particular day, etc.) what is the best way to help charge the batts with my generator?

Should I disconnect the cc before I use my inverter/charger?
Can I leave the CC connected (as it's charging batts) at the same time I'm using the inverter/charger?

I'm asking because both my Prosine 2.0 inverter/charger and Flexmax 80 CC use multi-stage algorithms (bulk, absorb, float) to charge batts.
Will those algorithms conflict with each other when operating at the same time, into the same battery bank?

What is the best system for supplemental charging and when should it be done?

Hope that makes sense.

Thanks,

BB. · May 2015

In general, putting charge controllers in parallel is fine. The controller with the "highest set-point" wins.

Specifically, high charge rates for lead acid batteries can cause them to heat up. More or less, 13% rate of charge is about the maximum before you have to start taking steps to ensure that all is OK with the battery bank.

Certainly, charging a 20-25% rate of charge for a flooded cell lead battery under ~80% state of charge is not usually a problem. But, as the batteries go higher, the self heating/gassing (losses) increase. So, normally we would suggest a remote battery temperature sensor to reduce the chances of thermal run away--When the batteries get hot, charging voltage of the battery drops (as batteries get hot, their charging voltage goes down)--Causing the charge controller to "think" that more current/charging is needed. Repeat and overheat.

You certainly can set one charger to a lower voltage, put the AC charger+genset on a timer, etc. (install a remote battery temperature sensor on all charge controllers--if the controllers support this option) and such.

Or, you can use the genset to charge in the morning (before full sun on the array), and then let the solar panels+charger finish up the charge.

Charging with the genset when the sun is not up, and the batteries are in the 50% to 80% state of charge range is a good start. If you have heavy loads at the same time (well pumping, washing machine/vacuum/etc.)--Certainly you can run the genset to help the battery charging on poor sun days.

-Bill

Hairfarm · May 2015

Can you recommend a battery temp sensor?
What's the best way to "know" when to need a generator for supplemental charging?

Thanks for the great tips. I'll make the set points the same. I'll have to factor in a .004 lower voltage adjustment for every 10 degrees over 80 according to my manufacturer to prevent over charging. It will get roughly 115 degrees by my desert cabin.

thanks!

BB. · May 2015

For the Outbacks, you have their RBTS (which, I guess, can use one of the Mates if you have several Outback devices you can network).

RTS - Outback Power Remote Temperature Sensor

For the other chargers--It depends if they support RBTS or not... The Xantrex Prosine 2.0 should have come with a RBTS according to the manual:

http://www.xantrex.com/documents/Inverter-Chargers/PROsine-2/DS20080923_PROsine_2.0.pdf

If you do not have one, you might call Xantrex or the retailer you purchased the Prosine from and ask about ordering a replacement.

And, the typical flooded cell offset for temperature is -5mVolts per C per Cell -- Or, for 25 to 35 battery temperature increase would be:

-0.005 volts per C per Cell * 10C increase * 6 cells (for 12 volt battery) = -0.3 degrees C charging voltage offset (95F for 12 volt battery)

Knowing when to run the genset... You need to know the state of charge of your battery bank. A Hydrometer is the best tool (for flooded cell batteries).

You can use a Battery Monitor (like the Trimetric or Victron)... They are nice, not cheap, and you have to understand their limitations (they can drift over time).

There are voltage only battery monitors... I have never used one, so I cannot attest to their accuracy/usefulness:

MidNite Solar MNBCM Battery Capacity Meter
http://smartgauge.co.uk/smartgauge.html

You can see how battery voltage varies based on charging/discharge rates:

http://forum.solar-electric.com/forum/solar-news-reviews-product-announcements/solar-information-links-sources-event-announcements/4426-working-thread-for-solar-beginner-post-faq?p=194303#post194303

And, as always, understand the battery:

http://www.windsun.com/Batteries/Battery_FAQ.htm
http://www.batteryfaq.org/
http://batteryuniversity.com/

-Bill

mike95490 · May 2015

Generally if you log and monitor morning and evening voltages, you will know when they are low. Did the absorb cycle complete, did any float time happen ? On cloudy /rainy days, I run the genset and any solar is bonus power !

Hairfarm · May 2015

BB. wrote: »

You can use a Battery Monitor (like the Trimetric or Victron)... They are nice, not cheap, and you have to understand their limitations (they can drift over time).

There are voltage only battery monitors... I have never used one, so I cannot attest to their accuracy/usefulness:

MidNite Solar MNBCM Battery Capacity Meter
http://smartgauge.co.uk/smartgauge.html

-Bill

Thanks for the great info Bill. I usually copy and paste stuff like this into a printable doc to use at the cabin where there is no network.

Question - How can a battery meter give me an accurate reading while the CC is connected to the same battery bank. For example: if the CC controller is feeding 14.4 (Absorb) into my batteries, then that is what the meter will tell me, right? But if I were to disconnect the batteries from the CC and all loads, then the "true" voltage would reveal itself, correct? Well how does a voltage meter know what the true voltage is unless the batteries are isolated from the CC and loads. Make sense?

Hairfarm · May 2015

mike95490 wrote: »

Generally if you log and monitor morning and evening voltages, you will know when they are low. Did the absorb cycle complete, did any float time happen ? On cloudy /rainy days, I run the genset and any solar is bonus power !

Never thought about that way. Yes, the absorb has usually completed every day and then goes to float. So that is a good sign, right? But I have been super conscience of my loads since this is a new setup and not really hitting the batteries hard.

BB. · May 2015

It turns out that Lead Acid Batteries are near 100% efficient when measuring Amps*Hours (except when the battery is gassing, then the AH efficiency drops). So--The current shunt is measuring current, the meter is keeping track of time, and the meter can accurately measure the discharge state of the battery. And keep the track of the battery as it charges (and, I am sure, a table that takes care of the losses as the battery recharges--Higher battery voltage, less efficient charging). (the battery monitor is measuring charging voltage and some measure battery temperature).

More or less, the Battery Meter resets to 100% full if the battery is >14.5 volts for several hours (again, programmable).

If you have a cup of coffee and some time, Smart Gauge has good size white paper comparing a Amp*Hour meter vs their voltage based computer models:

http://smartgauge.co.uk/sgvahrs.html

You can decide if you want to try one of their meters, and/or understand the limitations with Amp*Hour meters.

-Bill

Hairfarm · May 2015

Bill,

I just realized that battery monitors use current shunts. I wrongly assumed that that they were wired directly to the battery + - terminals. Your explanation clears that up for me know. I'll read that white paper too.

thanks all.

Using charge controller and generator to charge batteries in unision.

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