profdant139 wrote:
I am surprised to hear that 13.5 volts won't even charge a semi-depleted battery -- we have found, after much real-life experience, that my 40 foot cable will do a fine job of charging a battery at around 13.9 volts.
13.5V even with a partially depleted battery can result in not much more than a trickle charge. It is the reason why some converters pick 13.2V-13.4V as "storage" mode which basically is just enough current to replace the batteries "self discharge" rate.
13.5V is just a small notch above storage charging.
Some converters will run 13.6V-13.8V as "absorption" stage which may result in a few hundred milliamps to many amps depending on your battery state of charge and how much current your charging source can provide resulting in some charge. But to fully charge the battery it could take days or weeks at 13.5V
profdant139 wrote:
I know this for a fact because we test our battery (at rest) with a voltmeter almost every evening, after the sun goes down. It's almost always at 12.7 volts. Even on a bad day (some clouds or partial shade), it's at 12.5. By contrast, in the morning, it tests out at about 12.3 or so.
12.7V is low, your not fully charging the battery at the end of the day.. Resting voltage of a typical Lead Acid 12V battery should be 12.8V or a bit higher. I have seen charts which claim 12.73+ is "fully charged", in reality 12.8V or a bit higher will be considered fully charged..
I realize I am splitting hairs but in reality of 12V batteries even a small voltage difference of .1V can make a difference.
profdant139 wrote:
The obvious solution to all of this is to bring both cables but then to deploy the 70 footer only if needed -- otherwise, the 40 footer will be preferable. The problem, though, is that the 70 footer is very bulky and heavy, and storage room is at a premium. So if it causes a significant voltage drop, and will not do a very good job of recharging the battery, then I'm better off without it.
Best thing to do is to test it, measure the current coming from your solar panel using both cables. Then decide if the 70ft wire run is worth messing with. There will be some reduction in charging current using the 70ft vs the 40ft. Only you can determine if that sacrifice is worth it to you.
One of the downsides with your panels from what I have seen is the controller being attached to the panel.. That is putting you at a disadvantage, it is handicapping your setup by regulating the panel voltage remotely and the long wire run acts like a resistor which reduces the voltage to the battery and that reduced voltage means the battery cannot draw as much current that the panel could supply.
Would be far better if the charge controller was close to the battery, this way it will result in a higher voltage being delivered to the battery for a longer period of time. That would maximize your panels full power rating.
To maximize your panel charging you have to minimize the voltage drop between the charging source and the battery.
