From the previous post, my answers are embedded.
I have 2 house batteries, and I typically use one to run the systems while keeping the other in reserve for starting the engine or for emergency. Both are AGM, deep cycle but rated for starting also. (I would prefer a dedicated, third starting battery, but I'm hoping not to add that project to the list.)
Adding a third to the system is easy. I recommend being able to switch it in/out easily so you can isolate it and put it in only when you have a true emergency. Make it easy to connect for charging though. Have one bank (battery) for the house and one for the engine. The third can be switched in if needed for the engine which presumably would be able to charge the other two if they are discharged.
Using the engine to charge the batteries:
1. If one battery is fully charged (or very nearly full) and one is 50% down, should I charge both or each one separately?
It does not matter. (With a small exception.) If one battery is charged and the other is discharged, when you connect them together with no charger they will begine to equalize their charge, which is, the discharged battery will begin to suck power from the charged battery. As soon as you put the charger on then the voltage on the system will be above both batteries and the charge will go from the charger to the batteries, with most of the current going to to the discharged battery. Batteries "suck" power, chargers do not "push" power. The discharged one will draw more current. After you remove the charger (turn off the engine etc) then the batteries will again start to equalize their charge if they are not both fully charged.
The one small exception is that if one battery is defective, then it could cause a problem. Usually battery cells fail "open" as opposed to "shorted". They don't draw a charge or store a charge. If a cell in a battery fails open then it will not affect the other battery. If a cell shorts, then it could prevent the other battery from charging. Extremely rare though.
2. If I leave them both in the charging circuit, do I risk damaging the full one, and how could the regulator possibly know the difference?
No risk of damage the charged one, assuming the regulator is working fine. A regulator controls the voltage to the batteries so they are not over charged. A float voltage is usually applied, after a battery is charged, to maintain the charge. This voltage is about 13.2 volts on a wet lead acid battery.
3. Will it take longer if I leave them both in the circuit, or perhaps will one not receive a full charge (because of how the regulator works, perhaps)?
No risk of one not getting a charge if both are connected. I am assuming that both batteries are of the same type (do not need to be the same size) and are both functional, that is, no cells are dead.
How a regulator works: It puts out a voltage. The battery "sucks" current from it. As long as current is available (alternator is fine and wires are large enough) then current will go into the battery. The battery charge increases and the current "sucked" by the battery decreases until it reaches a very low value when the battery is fully charged.
4. I have a wired-in voltmeter and ammeter, nothing sophisticated, and with these can I determine when (let's assume) a single battery is fully charged? (I have read that the voltage on a battery should be read after it has "rested" following a charge--something to do with equilibrium between the electrolyte, cathode and anode?)
When a battery is discharged it will draw a large amount of current when a charger is connected. As the battery's charge increases the amount of current it draws decreases. You can tell the state of charge by the amount being drawn. Also, the voltage value when the charger is off is a good indication. A small load should be applied. 12.8 volts is a good charge. 11.9 indicates a very low charge.
5. Wishing it were not true, but guessing it is... related to the last question: obviously the output of the regulator is above the battery's present voltage, so while charging, the reading on the wired-in voltmeter must be higher than the battery's voltage, right? so would I need to turn off the engine (or take the battery out of the charging circuit) and wait until the battery is rested before using the voltmeter reading to assess state of charge?
Correct. Put a small load on the battery also. Maybe 5 or 10 watts. A light is sufficient.
6. The simple question is, how can I know how long I should charge? and one at a time or both?
Both is fine.
Charge until the current drops to a couple of amps.
Charging too long, assuming the regulator is functional and not putting out too much voltage, does no harm. It might cause the liquid to go down a bit in wet batteries, but that is about all. You should check the liquid level of wet batteries every month or two if you are working them.
7. What RPM is best?
Check your amperage. Generally 1400-1800 RPM will get the alternator to full output. If you increase the RPM and the current does not go up then you don't need to increase the RPM. Idle is usually not enough.
8. Can I switch from 'Battery 1' to 'Battery 2' on the fly, while the engine is running, without damaging anything (the switch does not pass through 'off' to go from '1' to '2'... I think it passes through 'both'.. i'll have to check this)
If it passes through both then you are fine. You want a "make before break" switch.
9. If it is recommended to add a monitor for state of charge, is there one that does the trick while charging? high, med, or low install challenge?
Monitors are only as good as the "efficientcy factor" that you put into the monitor. That is the factor that gives the ratio of amount of charge in versus the amount of charge you get out. You get less out than you put in. It is about 1.1 for most batteries. If you charge and discharge the system a lot and monitor it to verify that the monitor is tracking the charge correctly then they are great. I can explain how to verify the efficiency factor is correct if you want.
I have a monitor on board and find that the efficiency changes as the batteries age. Also the shunt value (the thing used to monitor the current going in and out) changes as it ages and salt water gets on it. (I will encapsulate the next one I get in plastic. ) I use voltage under load to determine the state of charge of my batteries. When they get to 11.9 they have very little left. I can run them down more, but not much more. I know many people will think monitoring voltage is not as accurate but it does work for lead acid batteries pretty well.
Think of charge in a battery like honey in a jar. If you draw it out slowly then you will get almost all of the honey. If you draw it out fast (large load on the batteries) then you will leave a lot more in the jar. Monitors don't handle that very well. That is why it is best to get a larger battery bank. The load is a smaller percentage of the battery capacity and huge loads in relation to the battery bank don't exist. With a small battery, the same load will be large for the bank and you won't get all the capacity out.
I have 2 house batteries, and I typically use one to run the systems while keeping the other in reserve for starting the engine or for emergency. Both are AGM, deep cycle but rated for starting also. (I would prefer a dedicated, third starting battery, but I'm hoping not to add that project to the list.)
Adding a third to the system is easy. I recommend being able to switch it in/out easily so you can isolate it and put it in only when you have a true emergency. Make it easy to connect for charging though. Have one bank (battery) for the house and one for the engine. The third can be switched in if needed for the engine which presumably would be able to charge the other two if they are discharged.
Using the engine to charge the batteries:
1. If one battery is fully charged (or very nearly full) and one is 50% down, should I charge both or each one separately?
It does not matter. (With a small exception.) If one battery is charged and the other is discharged, when you connect them together with no charger they will begine to equalize their charge, which is, the discharged battery will begin to suck power from the charged battery. As soon as you put the charger on then the voltage on the system will be above both batteries and the charge will go from the charger to the batteries, with most of the current going to to the discharged battery. Batteries "suck" power, chargers do not "push" power. The discharged one will draw more current. After you remove the charger (turn off the engine etc) then the batteries will again start to equalize their charge if they are not both fully charged.
The one small exception is that if one battery is defective, then it could cause a problem. Usually battery cells fail "open" as opposed to "shorted". They don't draw a charge or store a charge. If a cell in a battery fails open then it will not affect the other battery. If a cell shorts, then it could prevent the other battery from charging. Extremely rare though.
2. If I leave them both in the charging circuit, do I risk damaging the full one, and how could the regulator possibly know the difference?
No risk of damage the charged one, assuming the regulator is working fine. A regulator controls the voltage to the batteries so they are not over charged. A float voltage is usually applied, after a battery is charged, to maintain the charge. This voltage is about 13.2 volts on a wet lead acid battery.
3. Will it take longer if I leave them both in the circuit, or perhaps will one not receive a full charge (because of how the regulator works, perhaps)?
No risk of one not getting a charge if both are connected. I am assuming that both batteries are of the same type (do not need to be the same size) and are both functional, that is, no cells are dead.
How a regulator works: It puts out a voltage. The battery "sucks" current from it. As long as current is available (alternator is fine and wires are large enough) then current will go into the battery. The battery charge increases and the current "sucked" by the battery decreases until it reaches a very low value when the battery is fully charged.
4. I have a wired-in voltmeter and ammeter, nothing sophisticated, and with these can I determine when (let's assume) a single battery is fully charged? (I have read that the voltage on a battery should be read after it has "rested" following a charge--something to do with equilibrium between the electrolyte, cathode and anode?)
When a battery is discharged it will draw a large amount of current when a charger is connected. As the battery's charge increases the amount of current it draws decreases. You can tell the state of charge by the amount being drawn. Also, the voltage value when the charger is off is a good indication. A small load should be applied. 12.8 volts is a good charge. 11.9 indicates a very low charge.
5. Wishing it were not true, but guessing it is... related to the last question: obviously the output of the regulator is above the battery's present voltage, so while charging, the reading on the wired-in voltmeter must be higher than the battery's voltage, right? so would I need to turn off the engine (or take the battery out of the charging circuit) and wait until the battery is rested before using the voltmeter reading to assess state of charge?
Correct. Put a small load on the battery also. Maybe 5 or 10 watts. A light is sufficient.
6. The simple question is, how can I know how long I should charge? and one at a time or both?
Both is fine.
Charge until the current drops to a couple of amps.
Charging too long, assuming the regulator is functional and not putting out too much voltage, does no harm. It might cause the liquid to go down a bit in wet batteries, but that is about all. You should check the liquid level of wet batteries every month or two if you are working them.
7. What RPM is best?
Check your amperage. Generally 1400-1800 RPM will get the alternator to full output. If you increase the RPM and the current does not go up then you don't need to increase the RPM. Idle is usually not enough.
8. Can I switch from 'Battery 1' to 'Battery 2' on the fly, while the engine is running, without damaging anything (the switch does not pass through 'off' to go from '1' to '2'... I think it passes through 'both'.. i'll have to check this)
If it passes through both then you are fine. You want a "make before break" switch.
9. If it is recommended to add a monitor for state of charge, is there one that does the trick while charging? high, med, or low install challenge?
Monitors are only as good as the "efficientcy factor" that you put into the monitor. That is the factor that gives the ratio of amount of charge in versus the amount of charge you get out. You get less out than you put in. It is about 1.1 for most batteries. If you charge and discharge the system a lot and monitor it to verify that the monitor is tracking the charge correctly then they are great. I can explain how to verify the efficiency factor is correct if you want.
I have a monitor on board and find that the efficiency changes as the batteries age. Also the shunt value (the thing used to monitor the current going in and out) changes as it ages and salt water gets on it. (I will encapsulate the next one I get in plastic. ) I use voltage under load to determine the state of charge of my batteries. When they get to 11.9 they have very little left. I can run them down more, but not much more. I know many people will think monitoring voltage is not as accurate but it does work for lead acid batteries pretty well.
Think of charge in a battery like honey in a jar. If you draw it out slowly then you will get almost all of the honey. If you draw it out fast (large load on the batteries) then you will leave a lot more in the jar. Monitors don't handle that very well. That is why it is best to get a larger battery bank. The load is a smaller percentage of the battery capacity and huge loads in relation to the battery bank don't exist. With a small battery, the same load will be large for the bank and you won't get all the capacity out.
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