Charging sealed gel cell batteries from an alternator

Saltrock · September 2018

I am installing a 120 volt house power AC system in my conversion van and would like some feed back from others here who may have or may know what I am attempting to do. I have 2 Eaton VRLA sealed gel batteries that will be connected in parallel and are 75AH ea with a float voltage of 13.5-13.8 volts. The total connected continuous load will be close to 300-400 watts, (while I am camping). My plan is to install a 2000watt pure sine inverter as I want to have the capacity to run short term larger loads if needed. My intention is to install a parallel charging system supplied from the alternator on my 1996 Chevy conversion van. This will be wired thru a battery isolator or voltage sensing relay so as not to drain my start battery while the inverter is on and the van shut off. I will mount the battery tray under and outside of the van welded to the frame of the van. This is to keep air circulating around the batteries while I am driving down the road. This will also allow me to mount my inverter less than 3 feet from the batteries. The charging circuit cables for my house batteries will be about 10 feet long each and connected in parallel to the start battery. So here are my questions and concerns.
1.) I will be driving cross country for extended periods of time will this charging system damage my gel batteries?
2.) Will #2 fine strand welding cable be sufficient for the charging circuit, or is this overkill?
3.) What size fuse is needed for the charging circuit?
4.) What size fuse is needed for the house circuit to the inverter from the batteries?
5.) What would be a good inverter for this kind of application? I would like over voltage and under voltage protection.
6.) Where is a good place to source this kind of equipment?
7.) Thanx in advance for sharing your knowledge and experiences!!

mike95490 · September 2018

Saltrock said:

....The total connected continuous load will be close to 300-400 watts, (while I am camping). My plan......

Gel batteries are NOT suitable for anything but fire lighting backup and UPS backup power. They are designed for Float charging, not cycle use.

AGM batteries are different, they are sealed, can be charged rapidly and can supply large amounts of power.

But I call attention to your load estimate. It's not going to work.

300W continuous, in 12 hours is 3,600 watt hours. That is the Total capacity of 3, 90ah deep cycle batteries. To supply that, you would need at least 6, 90ah batteries, and that hits them with 50% discharge which will greatly shorten their life.

You need to accurately identify your loads, the hours required for each on, and what the recharge frequency is. (camp 3 days, drive 1)

Then we can try this again, but the way it is stated now, is going to cost a lot of $ to do it right.

Don't let me get started about the dangers of welding to vehicle frames, which ruins the factory heat treatment and softens the metal.

BB. · September 2018

Saltrock said:

I am installing a 120 volt house power AC system in my conversion van and would like some feed back from others here who may have or may know what I am attempting to do. I have 2 Eaton VRLA sealed gel batteries that will be connected in parallel and are 75AH ea with a float voltage of 13.5-13.8 volts. The total connected continuous load will be close to 300-400 watts, (while I am camping). My plan is to install a 2000watt pure sine inverter as I want to have the capacity to run short term larger loads if needed.

Note: a ~150 AH 12 volt lead acid battery bank really can support (reliably) a ~375 Watt maximum continuous load.

Yes, a good GEL battery can output high amounts of current (surge current)--But a useful amount of energy for a short period of time... For example, at best a 150 AH @ 12 volt "perfect" GEL battery bank could support 2,000 Watts for:

150 AH * 12 volts = 1,800 Watt*Hours of stored energy
1,800 WH * 1/2,000 Watt load * 0.85 typical AC inverter eff = 0.77 hours

And you should only discharge to ~50% capacity on a regular basis--You can discharge to 20% of capacity if you recharge right away--But that basically gives you something like 20 minutes of "useful" 2,000 Watt power.

My intention is to install a parallel charging system supplied from the alternator on my 1996 Chevy conversion van. This will be wired thru a battery isolator or voltage sensing relay so as not to drain my start battery while the inverter is on and the van shut off. I will mount the battery tray under and outside of the van welded to the frame of the van. This is to keep air circulating around the batteries while I am driving down the road. This will also allow me to mount my inverter less than 3 feet from the batteries. The charging circuit cables for my house batteries will be about 10 feet long each and connected in parallel to the start battery. So here are my questions and concerns.
1.) I will be driving cross country for extended periods of time will this charging system damage my gel batteries?
Depends... Most GEL batteries charge at ~14.2 volts maximum, and float at ~13.6 volts or so (roughly). As long as your charging system voltage is in that range, they should not be damaged.

However, your Van's alternator may have something to say about this. Most standard duty alternators can output >100 Amps for a few minutes, but then drop way down as they get hot. Also optimal battery charging voltage does depend on battery temperature. Your batteries may last 1-3 years before going bad--Is that OK? But, if they are not charged correctly, they can go bad in days or weeks.

2.) Will #2 fine strand welding cable be sufficient for the charging circuit, or is this overkill?
Probably OK, but you need to know the actual charging current, and length of cable. Also the charging voltage the alternator (to figure out voltage drop).
3.) What size fuse is needed for the charging circuit?
Fuses are sized to the AWG of the wiring (and temperature, conduit fill, insulation type, etc.). 2 AWG cable should be good to 95-130 Amps. Here is a NEC chart (usually pretty conservative):

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

4.) What size fuse is needed for the house circuit to the inverter from the batteries?
Depends on your inverter capacity and intended AC loads... The equation would look like:
2,000 Watts AC * 1/0.85 AC inverter eff * 1/10.5 volts battery cutoff * 1/0.80 NEC derating = 280 Amp minimum DC branch circuit wiring and fuse/breaker current rating.

5.) What would be a good inverter for this kind of application? I would like over voltage and under voltage protection.
Samlex and Cotek (same parent company) are nice inverters. There are many different families/models to choose from.

Note that the 10.5 (typical) battery cutoff voltage is for a truly "dead" battery... Some inverters have a programmable cutoff voltage--And if you can set 11.5 LVD (low voltage disconnect under load)--Your batteries will be happier.
6.) Where is a good place to source this kind of equipment?
I can point you to our host Northern Arizona Wind & Sun (out of Flagstaff Az):

https://www.solar-electric.com/

If you choose a different retailer/product bands, that is OK. The forum here is open to all for discussions.

7.) Thanx in advance for sharing your knowledge and experiences!!

-Bill

oil pan 4 · September 2018

Expect to get 10 to 20 amps charging with your vehicle alternator.

Charging sealed gel cell batteries from an alternator

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