24V battery charger recommendations needed

xallero

First time poster! HI. You guys rock. Been a casual reader of the content on this forum for a few years now.

I live near the border of Maine in New Brunswick, Canada.

Just bought a Champion 3000W generator. It's primary use will be for power tools, but I wish to make use of it as a back up for our battery-based system. Hence the quest for a suitable charger.

A quick rundown: I have 24V system. 1100W of panels. Four Rolls L16 6V FLA batteries @ 400aH each. A 24V 2000W Kisae inverter. And a 40 amp Epever MPPT charge controller.

I live off-grid year round. And my family and myself are relatively conservative in our power usage, a 24VDC refrigerator being our main draw.

I see the NOCO chargers are not lacking in publicity, probably because they are of good quality. But I'd prefer not to spend more money on a charger than I did on the generator. Doesn't seem right to me!

So I am ignorant about chargers and generators. All I know is, I probably should have just doled out more money for an inverter-charger from the hop. Now I need more components it seems, and the total expense of which I presume will be higher in the end.

ANYWAY. I need a 24V 20 amp charger? Less amperage or more? Not sure.

Any help would be wholly embraced. Thanks.

xa

BB.

Welcome to the forum Xallero,

I like to do an overall system math check--I am all for a "balanced" system design. Load->Battery Bank sizing; Battery bank->Charging requirements; loads+battery bank+location/sun->solar charging requirements.

Since I don't know your daily loads (by season)--We can skip to the battery bank itself and make some guesses.

A good place to start is a battery bank that supports 2 days of storage and 50% maximum "planned" discharge (makes daily solar charging "easier", longer battery life, good surge power for starting motors/pumps, etc.).

• 390 AH * 24 volts * 0.85 AC inverter eff * 1/2 days storage * 0.50 max discharge = 1,989 WH per day battery draw

That works well to support your DC fridge (may be around 1,000 WH per day), plus LED lighting, RV style water pump, cell phone charging, some laptop charging).

For a Flooded Cell Lead Acid storage battery (FLA), typically suggest around 5% rate of charge (for sunny weekend/non-winter, suggested EQ charging current of 2.5-5%) cabin use, 10%-13% for minimum for full time off grid (10% is usually the minimum recommended factory rate of charge), and 13% works as a cost effective maximum. For folks with gensets and/or poor winter harvest (or shading from trees, etc.), 20%-25% rate of charge maximum (with 13%+ rate of charge using a remote battery temperature sensor to keep bank from overheating).

• 390 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 734 Watt array minimum
• 390 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 1,469 Watt array nominal
• 390 AH * 29.0 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,909 Watt array "typical" cost effective maximum
  

And then based on your loads/seasonal usage/location. Fixed array facing south:
http://www.solarelectricityhandbook.com/solar-irradiance.html

Fredericton
Average Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 45° angle:
(For best year-round performance)

Jan	Feb	Mar	Apr	May	Jun
2.67	3.59	4.52	4.69	4.56	4.75
Jul	Aug	Sep	Oct	Nov	Dec
4.68	4.73	4.32	3.37	2.39	2.15

Lets toss the bottom three months (assuming using genset when weather/load support needed with poor sun):

• 1,989 WH per day * 1/0.52 off grid AC system eff * 1/3.37 hours of sun (break even October) = 1,135 Watt array

Suggest for base loads (loads that need power 24x7 like fridge and lights)... Use only 50% to 65% of predicted solar harvest (other "optional loads" when sunny weather/running genset):

• 1,100 WH per day (fridge+lights) * 1/0.52 off grid system eff * 1/3.37 hours of sun (Oct) * 1/0.65 base load fudge factor =966 Watt array "base load" 65% of harvest
• 1,100 WH per day (fridge+lights) * 1/0.52 off grid system eff * 1/3.37 hours of sun (Oct) * 1/0.50 base load fudge factor =1,255 Watt array based on 50% of base load harvest.

What is the estimated harvest for October (long term daily average) for your 1,100 Watt array:

• 1,100 Watts * 0.52 off grid system eff * 3.37 average October sun = 1,928 WH per day

Overall, you have a pretty well balanced system... You could justify adding some more solar to 1,469-1,909 Watts array total.

Now to genset charging. 10% and 20% charging numbers:

• 390 AH * 0.10 rate of charge = 39 amps
• 390 AH * 29.0 volts charging * 0.10 rate of charge = 1,131 Watts (10%)
• 390 AH * 0.20 rate of charge = 78 amps
• 390 * 29.0 volts charging * 0.20 rate of charge = 2,260 Watts (20%)

For a standard genset, suggest minimum loading at 50% (below 50% for standard genset (not inverter-generator), fuel economy falls (output power usage drops, fuel usage remains about the same GPH/LPH). And suggest not loading a gense with more than 80% of rated load for longer life, fewer problems (residential type gensets). Inverter-generators can run down towards 25% loading and still be "reasonably" fuel efficient.

• 3,000 Watts * 0.8 genset derating * 0.85 "average" battery charger eff * 1/29 volts charging = ~70 Amps maximum DC charging current from genset
• 3,000 Watts * 0.5 genset derating * 0.85 "average" battery charger eff * 1/29 volts charging = ~44 Amps maximum DC charging current from genset

So, around 44 to 70 amp @ 24 volt battery charger is a good range (decent fuel efficiency when bulk charging, not overheating genset/alternator/wiring during hours of battery charging).

The minimum suggested battery charger output--Would be around 5% rate of charge:

• 390 AH * 0.05 rate of charge = 19.5 Amps (so you can EQ batteries when needed)

To charge from 50% to 80% your battery bank, a 5%/10%/20% battery charger would take:

• 80%-50% = 30% capacity recharge
• 30% / 5% rate of charge = 6 hours
• 30% / 10% rate of charge = 3 hours
• 30% / 20% rate of charge = 1.5 hours

And, based on your 3kW genset, a 5% rate of charge will use (very roughly) 2x more total fuel than 10%+ rate of charge (genset+battery charger not very fuel efficient when well below ~44 Amp/11% rate of charge).

Suggest a good genset usage of charging battery bank from 50% to 80% when you are there... And charging to near 100% when locking up for winter (if you leave solar charging system on and have sun for a week after you leave, solar could take 80% to 90% State of Charge to "full" battery bank (batteries should not be stored below 75% SoC, however daily cycling between 50% and 80% SoC is OK--Just reach >90% SoC once a week to once a month). FLA batteries need recharging about once a month if stored at 25C/77F... At 0C/32F or below, can go 4 months (or longer if colder) without charging.

A suggestion is a 1% to 2% rate of charge solar panel(s) hung on side of building/vertical tilt/etc. to shed snow and avoid driving as much as practicable to help float batteries for long winter. All other loads/AC inverter/etc. should be turned off--Especially if panels are snow covered all winter.

Battery chargers--There are ones that are good for bulk charging (charge at rated current for hours at a time without overheating). And there are (more expensive) charge controllers that can do bulk/absorb/float/temperature compensation better (cold FLA battery bank need higher voltage when they are cold/very cold for full charging).

An example of a "simple" bulk charger (good for your 50% to 80% charging:

https://www.solar-electric.com/ioten25amp24.html
https://www.solar-electric.com/ioen40amp24v.html

And an example of a very nice battery charger (does everything, and has PFC--Power factor corrected AC input--"simple" battery chargers are notorious for having "poor power factor"--AC loading current is higher, but power usage is "relatively" lower--For smaller gensets you can run larger PFC based battery chargers vs the non-PFC versions).

https://www.solar-electric.com/xantrex-804-2420-truecharge2-20-amp-battery-charger.html
https://www.solar-electric.com/xantrex-804-2430-truecharge2-30-amp-battery-charger.html

Lots of stuff here... Before going into exactly what you want/need for a battery charger--Your thoughts and/or correction for my guesses?

-Bill

mike95490

With solar, using a simple automotive ( Truck ) 24V 40A charger would work OK for a couple hours to pump some juice into the batteries,
You will be monitoring this - right, not starting the generator and then going on a 5 hour trip to town ?

With chargers, you need to be aware of their power factor and how much of a load they present to the generator.   This load would be constant for an hour+ and can over heat a marginal generator.   24V @ 40A = 1kw.  Allowing 50% for bad PF, 2kw should be within the range of a 3kw generator

When I charge from the generator, I charge until the amps begin to decrease, and then I shut down and allow the solar to finish the charge and save fuel

littleharbor2

Look into Iota chargers. They are designed for your type of off grid usage. The forum host , Northern Arizona Wind and Sun, carries them.

xallero

Many thanks to BB., mike95490, and littleharbor2 for the valuable information and guidance.

I found a 40 amp charger by Progressive Dynamics through my local solar retailer. Feel free to check it out: https://www.thecabindepot.ca/collections/progressive-dynamics/products/progressive-dynamics-40-amp-24v-converter-charger-20a-plug

xallero

I might go with this 50 amp charger instead of the one previously linked.

Plus I have a follow-up question: How do I integrate a charger/converter into my system?

It's presume I would run an extension cord from my genset to the charger that I mount inside the house. But do these chargers include an AC input?

And do I run some 8awg red and black wires from battery bank terminals right to the charger?

BB.

As always, refer to the installation/user manual for your products (I think this is the one):

https://powermaxconverters.com/wp-content/uploads/2018/05/PM3-24V-LK-Manual-2018.pdf

I like to use NEC wiring Ampacity chart as it is a bit more conservative:

https://lugsdirect.com/WireCurrentAmpacitiesNEC-Table-301-16.htm

For battery charging where you can have hours of maximum current (charging battery from 20% to 80% or so State of Charge), I like to use the NEC 1.25 or 1/0.80 derating. Assuming 50 Amp continuous battery charging current:

• 50 amps * 1.25 NEC branch circuit derating for continuous current = 62.5 Amps minimum rated current

or 65 amps @ 75C or better rated insulation -> 6 AWG suggested.

The other thing to watch for is voltage drop... Too much voltage drop, and the charger is "reading" the wrong battery voltage. You want the converter/charger to "see" the accurate battery bank voltage to within 0.1 to 0.2 volts @ 24 volts...

Using a simple voltage drop calculator for 4 feet (one way run for this calculator) @ 50 Amps and less than 0.2 volt max drop:

https://www.calculator.net/voltage-drop-calculator.html?necmaterial=copper&necwiresize=4&necconduit=pvc&necpf=1&material=copper&wiresize=0.4066&resistance=1.2&resistanceunit=okm&voltage=29&phase=dc&noofconductor=1&distance=4&distanceunit=feet&amperes=50&x=0&y=0&ctype=nec

Result

Voltage drop: 0.20
Voltage drop percentage: 0.68%
Voltage at the end: 28.8

So 6 AWG and 4 feet or less wire run--You are good.

This charger family appears to have a 15 Amp @ 120 VAC standard 3 prong plug on a "short" cord.

The cable/extension cord--You want a nice heavy one... This charger will draw around 15 amps @ 120 VAC at maximum output (no specs--Just an estimate). At least 14 AWG, and probably 12 AWG if you are running (for example) a 50 foot cord. (you can use the above voltage drop calculator for 12 AWG @ 50 feet @ 15 amps for a 2.5% or 3 volt drop @ 120 VAC--Typically we use 3% maximum voltage drop to keep losses "low").

Connect the output of the Charger to the common battery bus connections in parallel with the rest of the chargers (solar, etc.) and your loads.

If you have any parallel battery connections (say 4x 6 volt @ 200 AH batteries in series x 2 parallel strings for 24 volts @ 400 AH battery bank), this site does a nice job of documenting the wiring to ensure that the battery strings share the current during charging/discharging equally:

http://www.smartgauge.co.uk/batt_con.html

-Bill

xallero

Many thanks, Bill. Tremendously helpful information.

Marc Kurth

FWIW: I used 12v, 100 amp Power Max chargers in my shop for quite a while and they worked great. (I ran them in parallel to get 200 amp charge current for our deep discharge recovery process.)

https://powermaxconverters.com/wp-content/uploads/2018/08/PMBC-ADJ-Info-Sheet_NP.pdf

I loaned them to a certain battery manufacturer and didn't get them back. 

Marc

xallero

@Marc Kurth

Interesting. I'm curious to see how my 24V 50 amp Power Max performs.

Do you know how I'm supposed to ground the unit? I have a lead acid battery bank, and the electrician that hooked everything up used a grounding rod. I'm not sure how to ground the charger to that connection. Where else could I attach a ground wire?