Powering a block heater from a second battery?

VW cat · Post by **VW cat** » Sat Sep 27, 2008 5:34 am

Did anyone ever make this work? I was wondering about this idea as a way to get a car going that is stranded in one of those cold parking lots in January. Saw a picture of one posted somewhere....the guy had a battery in the back, not sure how he wired it up or how well it worked out....Phil

82vdub · Post by **82vdub** » Sat Sep 27, 2008 8:28 am

I've though of that very idea, but never implemented it. You would need to isolate the additional battery so that you don't drain your main battery when you are heating the coolant. Plus, you would also need an easy method of energizing that battery into the charging loop when the car is running. Other than that, I think it's definately do-able with the right equipment.

VWCaddy · Post by **VWCaddy** » Sat Sep 27, 2008 9:11 am

A basic battery isolator would work, or a solenoid that comes on when the engine is running to connect the two batteries for charging. I have the latter setup in my 4x4 and it works well:
- http://www.4crawler.com/4x4/CheapTricks ... html#Parts

Vincent Waldon · Post by **Vincent Waldon** » Sat Sep 27, 2008 11:06 am

The trick would be finding a block heater that will run on 12V....using an inverter to get to 110 would cost a lot of efficiency.

And the actual power requirements need to be factored in. A 400W 12V heater will draw 400/12 or 33 amps... a 300W heater will draw 25 amps... so you'd want heavy duty wiring and a large deep-cycle battery as found on RVs. An RV battery isolator would make the charging system automatic.

surfcam · Post by **surfcam** » Sat Sep 27, 2008 1:04 pm

I agree with Vince but a did a little research and all I could find was 12 volt stick on oil pan heater. Someone posted a set-up with an inverter on this message board. It had two deep cycle AGM batteries and an inverter hooked to the block heater. He claimed it worked alright. I see some issues that you would have to deal with. I think you would need some kind of restriction on the charging circuit to stop the alternator from burning out. It would be better to charge the auxiliary batteries with 110. If you had to stay longer and the batteries could loose there jam. Maybe Vince could figure in his head the window of starting opportunity a person has.

joat · Post by **joat** » Sat Sep 27, 2008 4:21 pm

drill the block / tap for GLOW PLUGS ( 3 or 4 or more) / then hook up the 12V battery ... no inverter or special 120V circuits.

Vincent Waldon · Post by **Vincent Waldon** » Sat Sep 27, 2008 6:22 pm

Interesting idea. From a design perspective a couple of challanges would be:

- they are gonna draw 50- 60 Amps
- they aren't particularly designed for continuous duty.

tylernt · Post by **tylernt** » Sat Sep 27, 2008 8:34 pm

Vincent Waldon wrote:Interesting idea. From a design perspective a couple of challanges would be:

- they are gonna draw 50- 60 Amps
- they aren't particularly designed for continuous duty.

Don't TDIs have a glow plug in the coolant flange to help speed engine warmup?

I don't think continuous duty would be a problem as long as they weren't allowed to overheat. Duraterms are designed to handle the 5-minute afterglow cycle on Mk2s, after all.

Let's see, a 60Ah deep-cycle battery would be able to feed 3 glow plugs (one between each cylinder) for an hour before reaching 60% discharged. My 110VAC block heater seems to work best when left on for 1.5 to 2 hours, so a 100Ah deep-cycle would probably work.

So... you guys know that Webasto makes diesel-fired coolant heaters, right?

jason · Post by **jason** » Sat Sep 27, 2008 9:06 pm

The newer VW diesels (like my '98 Beetle) have 3 glow plugs in the coolant attached to the output of the head to help heat the interior when it's cold and I'm pretty sure they can run quite a long time, so you could possibly use those for a 12 volt system. If you were to use an inverter to power the built in heater, bank on pretty bad efficiency for most cheap ones; so a 400 watt AC block heater will draw 40 amps DC (400/10) instead of 33.3 amps (400/12).

I've used both isolators and solenoids to charge deep cycle batteries in campers, both worked fine. The isolator cost ~$60 and the continuous duty solenoid cost ~$20 from a place that works on semis, don't try to use a starter solenoid because even though they look the same they will burn out in short order. The nice thing about the solenoid is if you have at least enough power to click the solenoid from the main battery you can basically jump your starting battery without even popping the hood, with an isolator you need to jumper the posts on it or remove the battery and bring it up to the front of the car. The bad thing about the solenoid is when you have a discharged deep cycle, a cold engine, and a cold but charged starting battery, as soon as you click on the ignition the solenoid connects the starting battery (which is putting out less power because its cold) to the the discharged deep cycle, charging it at the expense of starting the vehicle. Of course you could have a momentary switch to use for those rare occasions or a timer click on the solenoid a minute after power is applied.

I think the main problem with battery based block heaters is weight and storage, I'm guessing at least 3 105 amp hour deep cycle lead acid batteries @ 55 lbs a piece to make a working system. 315 amp hours may seem like a lot, but you don't want to bring a deep cycle battery below 50% of it's rated capacity (depth of discharge (DOD)) very often because you shorten it's lifespan dramatically. Also you don't really get 315 amp hours when you up the current, the ratings on deep cycles are for current discharge over 20 hours @ 80 degrees Fahrenheit which would be a 5.25 Ah draw. The 80 degrees Fahrenheit is important too, at 0 degrees, the capacity drops to 50%. The batteries that I've used (Trojan 27TMX) are rated 25 amps for 175 minutes = 72.9 Ah and 75 amps for 45 minutes = 56 amps, at 80 degrees. So a 40 Amp load (400 watt AC block heater through an inverter) at 0 degrees with 3 batteries would last about 3 hours before dropping the batteries below 50% DOD. Of course my calculations could be way off and you could power it with a AA battery

What I've always wanted for emergency starts was a small 2 cycle gas generator, but they are pretty loud and not something I would want running for an hour or two in the parking lot at work. The other thing that would work (but are pretty expensive) are the diesel powered coolant heaters, basically a tiny on demand water heater.

Jason

joat · Post by **joat** » Sun Sep 28, 2008 9:24 am

Don't TDIs have a glow plug in the coolant flange to help speed engine warmup?

and to give heat to the cabin while the motor warms up

the coolant also prevents them from getting too hot and burning out.

blkboostedtruck · Post by **blkboostedtruck** » Mon Sep 29, 2008 8:37 am

what about implementing any of those 12v heaters for veg. oil?
Duane

VW cat · Post by **VW cat** » Tue Sep 30, 2008 5:34 am

I remember reading something about that somewhere...got a link? Phil

blkboostedtruck · Post by **blkboostedtruck** » Tue Sep 30, 2008 7:20 am

no i don't! maybe greaseworks.com may have something of that order?
Duane