Auto elec advice

Alternator output is at 14.08V, battery is drawing 1.1A from the alternator, W = V x A so 14.08 x 1.1 = 15.48W of power consumption from your alternator bus. The Amp Hour reading at the bottom is basically how many amps have been put into the battery over a period of time in Ah so if all goes well you will get something similar to this out of the battery when discharged but look up Peukert's law to see how high discharge currents give you less battery capacity and you may not get as much back out for high discharge rates.

Yep, volts are high and current is low so your battery is pretty well charged

so now gotta add mine, sorry for thread hijack


I wanna camp for 3 days and have solar replenish my car battery - I am only running a 39L Engel.
The solar puts out 21V.

Some people say bung that on the car battery as-is, just for a few hrs a day its all good at 21V
Most people use a regulator circuit to make it 14.4V or whatever.

I can see both sides.... I am sure the Engel is designed to accept 12-21V easily, and it could run on 12V most of the night and then 21V on the battery whilst being charged all morning...? chargers always put out more than the nominal voltage of the battery anyway. Is 21 too much...?

Do I just get another battery to run off and put solar on it for half a day whilst engel runs off car, then swap (ie: 2nd battery not in car,just next to the engel on the ground...)

21V will eventually kill your 12V battery Mark, you really should look at some sort of regulator for your solar. You might get away with something like Ian is suggesting and put two paralleled 6V cells in series with the 12V cell to give an effective 18V cell for charging (and then switch it all around to give 12V for driving the fridge). But remember that volts are only part of the equation, it's power output that rules the roost and if your solar doesn't put out enough power to actually put current into the batteries then it's a waste of time. The thing I hate with paralleled batteries without a good management/isolation relay is that if a cell fails in one battery, the good battery will feed the fault until it dies.

I never said it made economic sense, just said that it might work

Another key question is temperature, ie what fridge temp is required and what the ambient temperature is. I can run our car fridge pretty well indefinitely on a 120AH battery, with a 180w solar feeding it during the day. I adjust the fridge temp lower during the day, (when the sun is shining) to to minus 5 or 8, that keeps the frozen stuff at the bottom, (slowly defrosting meat etc) and the beer is perfect in the top. At night I set the temp of the fridge up to say 2 degrees...reduce the load on the battery at night... Also limit fridge openings, and keep it in the shade... 2 week camps on the beach up north in sept / oct no additional charging required... we also have a gas dometic fridge, which I reckon are awesome, but that's another story.....

^^^ Oh now you've done it Ian hahahaha
I wanted to get a cheap reg but didn't know which one would be cheap but work..... so the one posted above would be good
Now u make sense about wasting power. Especially as i only have 40W

Hmmm

The car is going to be run each day for 15min anyway so maybe just the solar and reg onto car battery as insurance.

A good regulator/charger is definitely the go, but the most important thing is to properly size your battery according to your load requirements and proposed charge/discharge cycle and then size your panels/regulator/charger to suit (remembering a rule of thumb for solar is roughly 5 hours at maximum panel output per day).

So if you have a load that needs 100Ah/day at 12V then you need a minimum 125Ah AGM battery (based on 80% discharge cycle) and you will need a solar system that will output the 100Ah back into the battery at 14.4V (or thereabouts) in about 5 hours. So this will mean an output of 20A (100Ah divided by 5 hours) at 14.4V which is 288W. This quick calc does not allow for losses and inefficiency, nor does it give you a fudge factor for a cloudy day. But you can easily correct by say doubling the battery/panel size to give two day's capacity between charge cycles and then apply an additional percentage to allow for inefficiency.

A big alternator like the oversized one in my Land Cruiser will pump out over 80A and not care if it's cloudy. In the case above, it will dump 100Ah back into the battery in just over an hour's driving. That's why I reckon if you're someone who is on the move a bit, use an oversized battery and alternator and don't bother with the solar. If you go to a camp for a week and don't drive the car then solar is the go.

I just saw your numbers above surfer62 and I reckon your setup will be fine, you'd probably get away with stopping somewhere for a couple of days as long as you manage the fridge and don't drink too much beer (hot cans will cost you more power to cool down) Something I should add is that you want some big cable to that battery too (if it's in your tray and not the engine bay) so you don't get a problem with voltage drop when it is charging when you're on the road.

Thanks paddles, owe you a beer