house battery capacity

[gothicsurf]

In the works is a simple 200W solar auxiliary battery system housed in a Toyota pickup with a camper shell.

To supplement power from the panels, I would like to have the alternator help top off the batteries whenever driving, and in that case I believe simple AGM/SLA are the only option without a DC-DC charger.

What I would like to know is what size battery I need to adequately handle the charge current from the alternator (60-70A). What I had planned for were 2 35Ah SLA batteries from Home Depot connected in parallel. Would 60-70A cause them to overheat? 60-70A is way more than the ideal 15A from a 200W MPPT solar array. Maybe a single 120 or 200 Ah would be better and stay cooler?

I plan on using a 200A circuit breaker (maybe 100A would work, but have read they pop at lower than spec), and would like to know the adequate gauge I can use with a 60-70A current; would 2 or even 4ga be fine? The circuit breaker being so large, would it adequately protect against a short?

 

[highdesertranger]

first off you are going about this backwards. lets start from the top,

1. figure out your electrical needs
2. size your battery to #1
3. Size your solar to #2

as far as you alternator charging. your alternator is self regulating there is no problem with it over charging. there might be a problem undercharging but the solar will take care of that.

that circuit breaker looks like a cheap chinese knockoff. I always recommend staying away from anything that advertises for "car audio". for quality components I like marine rated like Blue Sea, Bussman, etc.

highdesertranger

 

[Boondock Saint]

I went through this with my Jeep wrangler. I had two 105AH deep cycle batteries in the back of my rig and wanted to know if I could just charge them from the main vehicle voltage rail and if if that voltage would be too high etc. I called and spoke with an Exide engineer and was told no problem, can use the alternator voltages just fine, no damage etc.

I did that, I ran a cable in (long jumper cable with ends cut off) for my supply and used it to charge both batteries in parallel and supply voltage for other things in my rig. In the years I did that the batteries charged just fine and when I finally sold them as old batteries and I was not really up to hauling them around at 70lb each, they still worked perfectly fine.

You should add a relay / solenoid so that when you start your rig, it switches from the solar battery supply to the alternator instamagically.

Make really solid connections, don't overthink it, you'll be fine.

Stay away from the cheap chinese crap and buy the Blue Sea stuff from WA state.


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[Trebor English]

The engine computer on newer vehicles, the alternator built in regulator on older vehicles, set the alternator field current to get the desired voltage, usually about 14.4 for a few minutes after engine start then 13.8 to 14 later on.  The current rating  of the alternator is based on a special test set up where the field current is set by the testing equipment,  The RPM is high.  The temperature is monitored to stop the test when the alternator gets hot.  The voltage is monitored but maybe not part of the max amps test specification.  You will not get 60 amps into your battery.  While the alternator voltage is high the lead acid battery limits the charge current.  The chemistry of the acid film in contact with the lead plates is in control.  A bigger battery will be capable of accepting more current but that doesn't make more current flow.

My brother in law lives in Miami, FL.  He has a natural gas powered whole house generator that he got after hurricane Andrew left him with a whining wife for weeks on end.  When his house is powered by the FPL Turkey Point nuclear plant south of Miami his fridge uses no more power than when his natural gas generator is running, an hour every other week.  One has a much higher current rating than the other.

As to the battery temperature, two 35 amp hour batteries have more outside surface area than one 70 amp hour battery.  If you leave some space between, they should be cooler.  Snuggle the largest flat sides together they should be a bit warmer.  The difference won't be significant.  Get a kilobuck LiFePo4 battery and it will be cooler because they are more efficient.  Again, not significant.

On the internet you can find wire size ratings.  Search, I use duckduckgo.com, for the phrase "awg wire size chart" and look for the wikipedia page.  They have a chart that shows how many amps continuous the wire is good for depending on insulation temperature rating.  They also give 10 second and 1 second ratings for the purpose of blowing a fuse if there's a short circuit. 

Search for "voltage drop calculator" to find a page that can help with selecting wire size.  The fuse should be able to protect the size wire you are using.

If your house battery is down to 12 volts and the alternator has the starter battery at 14.4 volts you have a difference of 2.4 volts.  If you have AWG #10 wire it is 0.001 ohms per foot.  10 feet would make the resistance 0.01 ohms.  Theoretically,  I=E/R,  2.4V / 0.01 ohm = 240 amps.  However, your house battery voltage will instantly rise above 12.0 volts.  The alternator output will drop below 14.4 volts.  The resistance of your circuit breaker is significant.  The resistance of the return path through the steel chassis will be similar to the copper wire resistance. You won't get 40 amps much less 240.  AWG #10 is too small.  AWG #8 is pathetic.  AWG #6 is tolerable if it is short, real short.  Then you'll get 20 amps for several minutes until the engine computer cuts you back to 13.8 volts.  As a backup to a solar panel system you want something that can get you past multiple rain days and the occasional failure of the solar panel system.  Wires come loose killing the charge.  As the main source of charge you need to drive a lot with really fat wire or keep your electrical use down to just a cell phone and a light.

 

[gothicsurf]

For my needs I believe 70Ah of total capacity would be enough, but more wouldn't hurt. Just wasn't sure if 70Ah is too small to handle the load of the alternator, but if it's self-regulating, I guess that solves that.

You should add a relay / solenoid so that when you start your rig, it switches from the solar battery supply to the alternator instamagically.

Adding a solenoid to disconnect the MPPT while the car is running, or can both the starter battery and MPPT be connected to the house battery while the car is running .. I plan on putting a 200A relay between the two batteries that would open with absence of of 12V from the ignition ACC lead to prevent draining the starter, but still not sure if both charging systems can be connected simultaneously.

 

[B and C]

Both solar and alternator charging (shore too) play well together, no need to disconnect anything.

 

[MotorVation]

"For my needs I believe 70Ah of total capacity would be enough, but more wouldn't hurt. Just wasn't sure if 70Ah is too small to handle the load of the alternator, but if it's self-regulating, I guess that solves that."

I just want to make sure you know that with those batteries wired in parallel for 70ah, only half of that, or 35ah, is usable. If you go below 50% capacity you'll damage the batteries. That's pretty much the standard for all chemistries aside from lithium

 

[B and C]

I have 350 Ah of house battery that is charged through the alternator (as well as solar and shore). No worries, get as big a bank as you think you'll need.