Originally Posted by MarkJ
I want to buy a 100ah battery to add to the 2 x 55's and put in a smart charger, but the cost of that, installed (I can't do the smart charger) will be about $1,000. So I am trying to make do till I find a bank to rob
I have checked the voltage drop in the cable between the batteries and the socket I measure from and its quite a small drop, and allowed for. I can't get to the battery bank at sea, so one must make do
Mark - I don't want to be too simplistic as you probably know more than you think you know.
Adding more battery capacity simply means that you have longer between charges but without increased charging capacity (bigger alt, solar
etc.)you also have more charging time.
Consider the battery bank as a reservoir. You now have 110 ah and you plan to add 100 for a total of 210. A rule
of thumb is that you don't want to use more than 1/2 the available as going further shortens battery life. I personally think deep cycle batteries will go a lilttle further but stick with 1/2 for now. So, you currently have 55ah usable and plan to have about 105 usable.
Do a consumption
plan and you can calculate how long in time the 105ah will last.
Recharging - In simple terms the internal regulator has a sense wire that senses system voltage. What is key is where the sense line picks up the signal. A short run to the battery will cause charge voltage to be lower. because there is less resistance in the system on this short run and the sense wire sees a higher voltage close to the alternator.
A sense line that runs to the bus may sense a lower system voltage as there is more resistance between the alternator and the sense location.
The ridiculous example is the sense wire attached to the output wire of the alternator, effectively sensing voltage as it comes out of the alternator - not useful. The other end of the spectrum is so much resistance that the sense voltage is always low, the alternator amps up to max and can cook the batteries. A failed sense wire (open) can cause an alternator runaway.
When you add load at the same time as you are charging the problem of resistance is increased. After the short run to the battery you may see decent charge voltage but when you test the bus voltage at the panel or beyond (lighter plug) it is lower (13.0 etc). This is because resistance goes up with current
draw. Sensed close to the battery the system voltage will be higher and the internal regulator will be reading a higher voltage and dial back the charge rate(voltage).
This is why some old cars had dim headlights and so on. The alternator wasn't producing it's maximum because it was sensed near the battery. Resistance in old wires and connections (all the way to the headlights) dropped bus voltage even lower.
In your case I would suggest that the regulator sense is pretty close to the regulator - because that is usually standard. The cigarette lighter plug
is out at the end of the circuit and votage will read lower. Especially during a period where the battery is charging heavily and/or you have load on.
We have a very simple 3 (green, yellow, red) light indicator that we plugged into a lighter socket. We monitor
house until the green is off, the yellow is on and the red starts to flicker. Then we run the engine and the green light comes on immediately assuring us the system is charging.
Using accessories while charging. There is a limited amount of generation available. You probably have a 50 or 60 amp alternator. I would reckon about 60-70 percent effieciency and say that you could put in 40 amps per hour into the batteries. Your fridge is pulling about 48 amps per day and the other incidentals - unless you have heavy light usage - probably another 20.
Your two 30 minute charge periods is putting about 80 amps in and you are using 68. So you are balanced. If you add batteries and consume more you will have to add charge time to keep the reservoir full. If you have a total of 105 usable and you used it all you would have to up the charge time to maybe 1 1/2 hours a day.
Charging the computer while the engine is on is not problem in my opinion. We do it all the time. The inverter and the computer regulator will handle most votage variations.