Battery Array for Off-Grid In the Big City

[Anonymous]

Hey there, so glad I found this forum!

I’m part of a warehouse of artists and small creative businesses that is off the grid in the middle of San Francisco. Yes, you read that right.

We’re occupying a building where the wiring was stolen and so we’re operating with a gas generator.

We just started to get a battery array setup going so that we can be charging the batteries with the generator and minimizing our fossil-fuel usage. We’re also going to be getting some solar panels going as soon as we can (see my other post about that).

Our goal is eventually to be running completely off of solar power, which means we need a pretty beefy system as we’re talking about an 8000sf building here with power usage sometimes at 5-6kw (though usually under 1kw)

My questions here are regarding our battery array system.

I just ordered a 2500w semi-pure sine wave inverter that can be stackable. I’m getting reconditioned marine batteries from Interstate battery.

Two main questions:

1. I’ve been research and am a bit confused about what happens when you connect batteries in a series. I’ve heard that you can connect two 6v batteries to get a 12v current, but then also heard about connecting multiple 12v batteries, which is what we’d like to do. But then does it go to 24v, 36v, 48v, etc. as you connect more batteries? If so is there a way to connect them in a series and maintain the volts at 12 so the inverter?

2. Currently we just have one battery that we’re charging with a 10a car battery charger. I’m wondering what kind of battery charger we should get and what amperage to run it at. I know more amps means less deep of a charging cycle, so what’s a good middle ground? Is there a different recommendation for when we’re charging one battery versus four or five? Also, any recommendations for a charge controller that can handle high amps and watts?

Thanks

[elnav]

When wiring batteries in series the voltage adds up but current remains the same. When wiring batteries in parallel voltage remains the same but current are added together. Yes 24 an de 48 systems are possible but that requires different inverter. no you cannot use a 12V inverter on a higher voltage bank damage to the battery system is suer to result.

You are getting into some serious power system application from your description. I have been designing such systems for the last decade since 1996. Although the concept is fine you are likely to get into some real difficulty if you simply kluge stuff together based on random bits of “free advice” gleaned piecemeal from casual users. You need somebody with an engineering background and some real system experience.

You are not going to get good results using an automotive type taper charger regardless of its ampacity. 10 amps output is simply a big waste of gasoline. As soon as you pull more than 120 Watts from the inverter the charger is falling behind and the generator will be wasting fuel constantly. You need a 100 – 150 amp @12V charger for any sort of decent deep cycle battery bank. Optimally the charging rate for any given bank is between C/10 and C/4 for the wet cells. Some AGM can tolerate charging as high as C/3

Not only must the charger output be matched to battery bank ampacity but if you use an AC driven charger it must be power factor corrected and it should be matched so the generatos is running at close to optimum output.

For a system I designed for a yacht built in Turkey we had a custom built generator made in Oregon because none of the standard off the shelf generators in Europe or in North America was suitable. Another option is a DC generator. This is something you can build yourself. Several of my customers have used this appproach with excellent results. But I have to admit it is different enough that operator education is required. It is not necessarily intuitive.

Let me know if you need more help. You can send me direct email at this address 2elnav (at) netbistro.com remove the spaces and substitute the correct symbol.

[elnav]

@thebgw after mulling over your situation, I decided this situation is different enough while becoming more likely in the future that it deserved a chapter in the new book I am currently writing. Please get in touch directly. Email me – 2elnav (at) netbistro.com

Judging from GRIST magazine articles about urban farming and such, there is a lot of potential for this kind of application. It also poses some unique limitations that does not apply to rural off-grid residential situations.

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