Sunday, Mar 29, 2009 at 10:47
Hairy,
Firstly, regarding the Supercharge Allrounder. This appears to be a battery that’s useful as a deep cycle, while also being useful to start the engine. This must involve compromises because cranking batteries and deep cycle batteries are usually constructed differently. I don’t know the Allrounder but the spec sheets look good. The MRV70 for example is rated at 105 Amphours (Ah) while also having a Cold Cranking Amps (CCA) rating of 760. Both these numbers are good for a battery of that physical size.
While I like the idea of having a spare cranking battery on board, if it is really only going to be used for running the fridge, I’d go for a straight deep cycle battery and avoid any compromises involved in the dual purpose battery. Depends on how much you value that dual purpose capability and of course how much it’s going to cost you. If you do decide to go that way, suggest get the one which has the terminals the same way round as your starting battery – positive to the left (or right). That way you can easily replace the starting battery if the need arises. I think you’ll have a choice with the MRV70 and MRV70L.
“If my fridge draws 1Ah and my solar panel puts in 3, why wont it run indefinately? “
Your fridge doesn’t draw 1 Ah.
While it’s motor (compressor) is running it probably draws about 4 or 5 Amps. (My 40 litre Waeco draws about 3.8 amps. Don’t think your 60 litre uses the same small compressor). If we guess that the compressor runs about 1/3 of the time, then in round figures it will draw an average of about 1.5 amps. On average then, every hour it will require about 1.5 Amphours. In 24 hours it will require 36 amphours,…..etc. In practice, it depends very much on just how long the compressor runs – I’ve talked of 1/3 of the time, but if it’s 2/3 of the time, obviously we’ll need twice the amphours. Realisticly, your big Waeco probably needs about 40 amphours per day.
This is where the differences between a cranking battery and a deep cycle battery become important. The energy in the battery is measured in amphours and it’s that energy we want to run the fridge. For starting the engine though we aren’t interested in the total energy in the battery, just in how fast we can get some of it out. Hope that won’t take hours – we are really only concerned with getting lots of amps for a few seconds. The CCA (cold cranking amps) rating is important for a cranking battery, but for running a fridge we want lots of hours and peak amps isn’t important.
So, in 24 hours we need about 40 amphours to run the fridge. The 40 watt solar panel will provide a bit over 2 amps in strong sunlight if it’s aimed straight at the sun. On a clear day, you get strong sunlight for about 6 or 7 hours, useful weaker sunlight for a couple of hours before and after. So, multiplying amps by hours to get total daily energy, the 40W panel will provide maybe 15 to 20 amphours a day, much less under cloud or with partial shading. That’s about half what the fridge requires, so the rest (say 20 amphours) must come from the battery.
The battery should not be discharged too far if it is to have a good service life. It’s best not to take out more than about 2/3 of the rated capacity. So ideally, we can get about 70 amphours from a fully charged 100 amphour battery. From the above figures, that’s enough to supplement the solar panel for 3 or 4 days, which is about what you’ve been getting.
“Is there a simple....ball park figure...like if your Waeco draws 1Ah it will run for 12 hrs off a 25Ah battery or 50Hrs of a 100Ah battery?”
Ballpark – the fridge needs say 30 to 40 amphours per day. Under ideal conditions the 40W solar panel can supply 15 to 20 amphours, leaving the battery to supply another 15 to 20 amphours. From fully charged, the battery can supply up to about 90 amphours (before the fridge cuts itself off) or about 70 amphours if we value battery life. So without the solar panel’s help, the battery will run the fridge for about 2 days, or with the 40W panel, 3 or 4 days.
More ballpark – If you add an 80 W panel as described below, you could generate another 30-40 amphours per day of good sunshine. This is actually more than you need, so you can survive a few cloudy days, or be less fussy about aiming at the sun.
Now – regarding that extra panel.
Firstly, don’t be too concerned about the stereo draining your cranking battery too far. I wouldn’t expect it to draw more than 1 amp, or to run for more than a few hours each day, so maybe 5 amphours tops each day. In fact you’ll need a solar controller ($100+) to stop the 40W panel overcharging and damaging the cranking battery if the stereo is the only load.
If you do add an 80 watt panel to your 40 watt one, I’d suggest simply wire them in parallel (connect plus to plus, minus to minus) and use them as a single big panel to charge the auxiliary battery. With this much capacity you will need a controller to protect the battery from being overcharged. The connections could either be from panel to panel then to the controller, or both panels to the controller – electrically amounts to the same thing. I use a 60W panel plus an 85W panel this way and they can provide more energy than I need. It’s good practice to use fairly heavy cable (6mm twin is good) though this isn’t very critical since you can afford some voltage losses. (The panels can supply excess voltage to compensate.) If you are concerned about flattening your cranking battery, just arrange your wiring so that you can connect the output of the solar controller temporarily to the cranking battery if need be.
Hope that helps. A lot of words, so I hope I don’t confuse, but it’s an area where there is a lot of confusion already!
Cheers
John | J and V
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