AB switch "Both" with one discharged battery

[Delawaredave]

Hunter 27. AB switch and two batteries. Use battery 1 for starting, 2 for accessories.

Often hang out in mooring, using Battery 2 for accessories. So Battery 2 gets drawn down to very low voltage (meter says 3 volts ?)

When going out, I switch to Battery 1, engine starts right up.

QUESTION: if I then run on "both", are both batteries getting charged ? Or is only the lower voltage battery taking current until it gets to voltage of higher battery ?

While in "Both" can energy be "siphoned" from Battery 1 to Battery 2 ?

I'm concerned that the huge difference in voltages across the two batteries is not replenishing Battery 1 in "both" mode.

I could run on 1 for awhile to replenish battery 1, and then switch to 2 - but I don't know how long to run each - and I often forget.

I was hoping that "both" configuration had some "smarts" - where both batteries would be getting charged and battery 2 isn't "sucking" from battery 1.

Thanks !!!

 

[JK_Boston_Catalina310]

Hunter 27. AB switch and two batteries. Use battery 1 for starting, 2 for accessories.

Often hang out in mooring, using Battery 2 for accessories. So Battery 2 gets drawn down to very low voltage (meter says 3 volts ?)

When going out, I switch to Battery 1, engine starts right up.

QUESTION: if I then run on "both", are both batteries getting charged ? Or is only the lower voltage battery taking current until it gets to voltage of higher battery ?

If it is the typical switch then yes. It is a charge selector switch as well as a use switch. Also, you should never turn the switch when the battery is running.

While in "Both" can energy be "siphoned" from Battery 1 to Battery 2 ?

Yes. It will equalize the charge and leave you without juice to start if one bank is really low.

I'm concerned that the huge difference in voltages across the two batteries is not replenishing Battery 1 in "both" mode.

I could run on 1 for awhile to replenish battery 1, and then switch to 2 - but I don't know how long to run each - and I often forget.

I was hoping that "both" configuration had some "smarts" - where both batteries would be getting charged and battery 2 isn't "sucking" from battery 1.

Thanks !!!

It sounds like you want a system with an ACR. Check out my blog or even better, go to the Mainesail section of the featured contributors. He has a sticky on the 1/2/both/off switch.

 

[Benny17441]

You do not indicate what types of batteries you have. If both are the same and ideally both are deep cycle than why don't you alternate their usage. This will exercise both batteries the same way. I would suggest you you use battery (A) one outing to start the engine and run the accesories and for the next outing use battery (B) This way you have one fully charged battery in reserve should the one in use get to low to start the engine. If you are running the engine the battery getting the charge from the alternator is the one selected via the switch. If you select Both then both of them will receive charge. The system is not very smart but it just happens there are compensating factors. A discharged battery will accept a larger rate of charge than a fully charged one. The alternator will recognize a bank of batteries as a single battery and will energize or deenergize according the average voltage among them. To answer your question, the state of the charged battery will mask the actual state of the discharged one. A weak battery will draw the juice out of a fully charged one but that does not happen instantly. The weak battery has a rate at which it will accept charge and that will limit and extend the transfer to a good number of hours. Combining both for a few minutes will do very little to each. Now the smarts of the system comes from the chemistry in the batteries. With a steady charging current as provided by the alternator under the control of the regulator the more discharged battery of the two will receive most of the charge and will likely stop drawing from the other as the alternator puts out a higher voltage than the other battery can. Now if battery 1 is deeply discharged you may want to charge it by itself to allow the alternator to produce maximum charge. If you alternate battery use foe each outing the battery not in use should remain at near full charge and no need to involve it in the charging process. The charging process slows down as the battery reaches 90% charge so it will take from 3 to 5 hours to fully charge a battery so unless you want to motor that long I would urge to install an inboard electric multistage charger to operate from shorepower if at all possible. Such a charger will smartly charge both batteries simultaneously. As far as forgetting to switch batteries as you go along your system for remembering may be as good or unreliable as mine. For that reason I try to keep it in just one battery at a time. To inadvertedly draw down both batteries at once spells $$$ when calling for a jump. To that intent we carry a portable power pack as an emergency supply to start the engine. Tried not to get technical, I hope this helps.

 

[robertsapp]

OK, first, 3 volts? Are you kidding? If these are your typical 12 V batteries, you'll never want to discharge them below about 12.1 volts or so, and at 10.5 volts they're totally dead. If you were running accessory loads off that battery, the radio and lights or whatever else you were running would have long since died before you ever got to a voltage that low. So if the lights still work, you're not at 3 volts, there's something wrong with the measurement.

Next, the question about where the charge goes depends on whether your engine or charger is running or not. If you have one battery depleted, and one charged, and you put your A+B switch in Both, the charged one will try to charge up the dead one, PROVIDED there is no other charge source present. But if you switch to your starting battery, start your engine, and then switch to Both, the approx. 13.4 volts that your alternator should be pumping out will charge both batteries simultaneously, although the more depleted one should take most of the charge, as it will have the higher acceptance rate.

And unless there's something physically wrong with your battery switch, you can change back and forth at will from A to Both to B without hurting anything. But switching it to OFF while the engine is running will potentially blow the diodes in your alternator regulator.

 

[JK_Boston_Catalina310]

OK, first, 3 volts? Are you kidding? If these are your typical 12 V batteries, you'll never want to discharge them below about 12.1 volts or so, and at 10.5 volts they're totally dead. If you were running accessory loads off that battery, the radio and lights or whatever else you were running would have long since died before you ever got to a voltage that low. So if the lights still work, you're not at 3 volts, there's something wrong with the measurement.

Next, the question about where the charge goes depends on whether your engine or charger is running or not. If you have one battery depleted, and one charged, and you put your A+B switch in Both, the charged one will try to charge up the dead one, PROVIDED there is no other charge source present. But if you switch to your starting battery, start your engine, and then switch to Both, the approx. 13.4 volts that your alternator should be pumping out will charge both batteries simultaneously, although the more depleted one should take most of the charge, as it will have the higher acceptance rate.

And unless there's something physically wrong with your battery switch, you can change back and forth at will from A to Both to B without hurting anything. But switching it to OFF while the engine is running will potentially blow the diodes in your alternator regulator.

Mainesail has routinely advised not to switch your 1/2/both/off switch while the engine is running depending upon the type of switch you have. Mine specifically says not to switch while the engine is running. I will continue to follow Mainesail's advice.

 

[Maine Sail]

Mainesail has routinely advised not to switch your 1/2/both/off switch while the engine is running depending upon the type of switch you have. Mine specifically says not to switch while the engine is running. I will continue to follow Mainesail's advice.

If it is a "make before break" switch, most all marine battery switches for the last 25 years have been, it is fine. It is passing though OFF where the issues arise. With the number of alts with blown diodes I repair each year it is apparent people still have not heard about the dreaded "switching through off" conundrum..


Oh and 3V..????? Those batteries are now well past their prime... 50% of capacity should be the regular max depth of discharge if you want them to last more than a single season...

****Battery SOC at Resting Voltage and 80f (Source Trojan Battery)*****

100% = 12.73

90% = 12.62

80% = 12.50

70% = 12.37

60% = 12.24

50% = 12.10

40% = 11.96

30% = 11.81

20% = 11.66

10% = 11.51