6 Volt Battery

[formerFR]

since someone mentioned the confusing '50%' battery discharge 'rule'... I had an epiphany the other night.... at least about how to 'understand' this and it's relation to the SOV(state of charge) or Voltage Reading... so here goes:

-a fully charged 12v battery, or battery bank, is normally 12.80 volts 'at rest' (meaning that it has just been fully charged, it is no longer charging, and there are no devices drawing any current amps)... at least that's the 'normal' assumption - so we'll use 12.8.


but 'WHY' is 12.8 the 'number' for a fully charged 12v battery? Shouldn't it be 12.0 volts?

My epiphany was this:

The battery 'Level', or POWER output, must be equal to, or HIGHER, than the 12v device that it is powering, otherwise the 12v device is receiving LESS power than it is 'intended' to need. So it then makes sense that the battery should have at least that much...

BUT...

as the battery is drawn upon to power devices at this 12v level, for each device, it will certainly start to LOSE it's ability to output that much power, as time goes by, and depending on the number of items that are needing power. Without getting into watts and amps, lets just stay with the Volts for this conversation.

as the battery is drawn upon, it seems natural that it's own VOLTAGE level would then start to decrease, such as from 12.8 to 12.6 after some time, and so forth until at some point the battery's own Voltage level is actually BELOW that of the devices it is trying to power. Stay with me here....it's coming....


SO...

look at it like a waterfall, or better yet, a river's LOCKS that hold back water on one end until the gates open....THEN, the water starts to LEVEL out. The 'waterfall' from the high side feeds over into the lower side, until at some point, the two are then LEVEL.

Think about the battery VOLTAGE the same way:
- the Battery is at 12.8 Volts (the HIGH side of the river Locks)
- 12v devices start to use this power, or 'voltage' (the gates are slightly open....)
- the 12v devices continue to use the battery power (the gates are now full open, and the water is starting to become more LEVEL)
- the BATTERY Reaches 12.0 VOLTS, now 'even' with the 12v devices drawing from it

I guess a real 12.v output battery needs to be HIGHER own it's own to do it's work...hmmmm. o.k.

so, is this why 12.0Volts for a 12v Battery equals 50% of the 'usable' power?
Maybe...

but, to simplify, let's drop the '50%' term, and just say that a 12v Battery is fully discharged at 12.0Volts, for all 'normal' usage. Yes, you could say that it could go lower, but since the 'acceptable' low point is considered to be a 'safe' 50%, then why not just say 12.0Volts, instead?


another request:
Since most RVs have at least a battery 'VOLTAGE' reading, and not a SOC(state of charge, such as 50%), it would seem that this could be more understood by the average RVing public.
If your battery gets to 12.0volts, it's time to recharge it, simple.

also:
Do I then want my AGS(automatic generator starter) to trigger at 12.0volts? well, yes, and no. My Magnum AGS's protocol is to start at the set trigger voltage AFTER the battery voltage has fallen below the set Volts for at LEAST two continuous minutes. I might then want to set my AGS at 12.1 Volts, so that by the time the two minute 'rule' triggers the AGS to prime and start the Generator, allowing also for the 20-30seconds for the ATS(auto transfer switch) to respond, to allow the CHARGER to start recharging the battery bank.

BUT....
Some smaller RVs only have a 4-stage 'light' indicator for battery voltage, which may make this whole conversation tough for them, because I don't know 'what' it is indicating. Certainly not a true SOC(State of charge), as it probably has no function to do that. I'm not sure if that reading is each stage being 25% of the 'whole' battery, meaning that two stages would only indicate 12.8 divided by 2, or 6.4Volts(?)
or is it an assumption that each 'stage' is 25% of the 50% discharge max?


thanks for listening

[rk06382]

Originally Posted by formerFR

since someone mentioned the confusing '50%' battery discharge 'rule'... I had an epiphany the other night.... at least about how to 'understand' this and it's relation to the SOV(state of charge) or Voltage Reading... so here goes:

-a fully charged 12v battery, or battery bank, is normally 12.80 volts 'at rest' (meaning that it has just been fully charged, it is no longer charging, and there are no devices drawing any current amps)... at least that's the 'normal' assumption - so we'll use 12.8.


but 'WHY' is 12.8 the 'number' for a fully charged 12v battery? Shouldn't it be 12.0 volts?

My epiphany was this:

The battery 'Level', or POWER output, must be equal to, or HIGHER, than the 12v device that it is powering, otherwise the 12v device is receiving LESS power than it is 'intended' to need. So it then makes sense that the battery should have at least that much...

BUT...

as the battery is drawn upon to power devices at this 12v level, for each device, it will certainly start to LOSE it's ability to output that much power, as time goes by, and depending on the number of items that are needing power. Without getting into watts and amps, lets just stay with the Volts for this conversation.

as the battery is drawn upon, it seems natural that it's own VOLTAGE level would then start to decrease, such as from 12.8 to 12.6 after some time, and so forth until at some point the battery's own Voltage level is actually BELOW that of the devices it is trying to power. Stay with me here....it's coming....


SO...

look at it like a waterfall, or better yet, a river's LOCKS that hold back water on one end until the gates open....THEN, the water starts to LEVEL out. The 'waterfall' from the high side feeds over into the lower side, until at some point, the two are then LEVEL.

Think about the battery VOLTAGE the same way:
- the Battery is at 12.8 Volts (the HIGH side of the river Locks)
- 12v devices start to use this power, or 'voltage' (the gates are slightly open....)
- the 12v devices continue to use the battery power (the gates are now full open, and the water is starting to become more LEVEL)
- the BATTERY Reaches 12.0 VOLTS, now 'even' with the 12v devices drawing from it

I guess a real 12.v output battery needs to be HIGHER own it's own to do it's work...hmmmm. o.k.

so, is this why 12.0Volts for a 12v Battery equals 50% of the 'usable' power?
Maybe...

but, to simplify, let's drop the '50%' term, and just say that a 12v Battery is fully discharged at 12.0Volts, for all 'normal' usage. Yes, you could say that it could go lower, but since the 'acceptable' low point is considered to be a 'safe' 50%, then why not just say 12.0Volts, instead?


another request:
Since most RVs have at least a battery 'VOLTAGE' reading, and not a SOC(state of charge, such as 50%), it would seem that this could be more understood by the average RVing public.
If your battery gets to 12.0volts, it's time to recharge it, simple.

also:
Do I then want my AGS(automatic generator starter) to trigger at 12.0volts? well, yes, and no. My Magnum AGS's protocol is to start at the set trigger voltage AFTER the battery voltage has fallen below the set Volts for at LEAST two continuous minutes. I might then want to set my AGS at 12.1 Volts, so that by the time the two minute 'rule' triggers the AGS to prime and start the Generator, allowing also for the 20-30seconds for the ATS(auto transfer switch) to respond, to allow the CHARGER to start recharging the battery bank.

BUT....
Some smaller RVs only have a 4-stage 'light' indicator for battery voltage, which may make this whole conversation tough for them, because I don't know 'what' it is indicating. Certainly not a true SOC(State of charge), as it probably has no function to do that. I'm not sure if that reading is each stage being 25% of the 'whole' battery, meaning that two stages would only indicate 12.8 divided by 2, or 6.4Volts(?)
or is it an assumption that each 'stage' is 25% of the 50% discharge max?


thanks for listening

My head hurts after reading that.

That is why I installed a Victron BMV-712. It calculates the State of charge by counting the amps used vs charged factored by the Peukert exponent.

Battle Born LiFePO4 stay above 13v most of the time. I had no idea how much power I had available without the BMV-712. I can also link to the BMV-712 with bluetooth from my house living room with my smartphone.

Above: With a 2.5 amp load. 13.11v
Below: With the FR3 engine alternator charging at 93 amps. 13.59v


Battle Born LiFePO4 are safe to discharge down to 20% or less without shortening the 3,000 life cycles of the battery. The can be charged at 50% of the 100ah rating. I have four so I could recharge at 200 amps.

[CampinGpa]

Originally Posted by formerFR

since someone mentioned the confusing '50%' battery discharge 'rule'... I had an epiphany the other night.... at least about how to 'understand' this and it's relation to the SOV(state of charge) or Voltage Reading... so here goes:

-a fully charged 12v battery, or battery bank, is normally 12.80 volts 'at rest' (meaning that it has just been fully charged, it is no longer charging, and there are no devices drawing any current amps)... at least that's the 'normal' assumption - so we'll use 12.8.


but 'WHY' is 12.8 the 'number' for a fully charged 12v battery? Shouldn't it be 12.0 volts?

My epiphany was this:

The battery 'Level', or POWER output, must be equal to, or HIGHER, than the 12v device that it is powering, otherwise the 12v device is receiving LESS power than it is 'intended' to need. So it then makes sense that the battery should have at least that much...

BUT...

as the battery is drawn upon to power devices at this 12v level, for each device, it will certainly start to LOSE it's ability to output that much power, as time goes by, and depending on the number of items that are needing power. Without getting into watts and amps, lets just stay with the Volts for this conversation.

as the battery is drawn upon, it seems natural that it's own VOLTAGE level would then start to decrease, such as from 12.8 to 12.6 after some time, and so forth until at some point the battery's own Voltage level is actually BELOW that of the devices it is trying to power. Stay with me here....it's coming....


SO...

look at it like a waterfall, or better yet, a river's LOCKS that hold back water on one end until the gates open....THEN, the water starts to LEVEL out. The 'waterfall' from the high side feeds over into the lower side, until at some point, the two are then LEVEL.

Think about the battery VOLTAGE the same way:
- the Battery is at 12.8 Volts (the HIGH side of the river Locks)
- 12v devices start to use this power, or 'voltage' (the gates are slightly open....)
- the 12v devices continue to use the battery power (the gates are now full open, and the water is starting to become more LEVEL)
- the BATTERY Reaches 12.0 VOLTS, now 'even' with the 12v devices drawing from it

I guess a real 12.v output battery needs to be HIGHER own it's own to do it's work...hmmmm. o.k.

so, is this why 12.0Volts for a 12v Battery equals 50% of the 'usable' power?
Maybe...

but, to simplify, let's drop the '50%' term, and just say that a 12v Battery is fully discharged at 12.0Volts, for all 'normal' usage. Yes, you could say that it could go lower, but since the 'acceptable' low point is considered to be a 'safe' 50%, then why not just say 12.0Volts, instead?


another request:
Since most RVs have at least a battery 'VOLTAGE' reading, and not a SOC(state of charge, such as 50%), it would seem that this could be more understood by the average RVing public.
If your battery gets to 12.0volts, it's time to recharge it, simple.

also:
Do I then want my AGS(automatic generator starter) to trigger at 12.0volts? well, yes, and no. My Magnum AGS's protocol is to start at the set trigger voltage AFTER the battery voltage has fallen below the set Volts for at LEAST two continuous minutes. I might then want to set my AGS at 12.1 Volts, so that by the time the two minute 'rule' triggers the AGS to prime and start the Generator, allowing also for the 20-30seconds for the ATS(auto transfer switch) to respond, to allow the CHARGER to start recharging the battery bank.

BUT....
Some smaller RVs only have a 4-stage 'light' indicator for battery voltage, which may make this whole conversation tough for them, because I don't know 'what' it is indicating. Certainly not a true SOC(State of charge), as it probably has no function to do that. I'm not sure if that reading is each stage being 25% of the 'whole' battery, meaning that two stages would only indicate 12.8 divided by 2, or 6.4Volts(?)
or is it an assumption that each 'stage' is 25% of the 50% discharge max?


thanks for listening

Thank you well put

[pgandw]

Actually it has nothing to with the battery voltage relative to the 12V devices. The voltage of a lead acid battery is the strength of the sulfuric acid solution in the cell times the number of cells.

A fully charged cell is 2.12 volts x 6 cells = 12.72 volts. These figures change slightly with temperature. As the solution gets weaker (battery discharges), the voltage per cell drops. The 12.0/12.1 50% SOC point (exact number depends on temp) is an arbitrary "do not go below" recommendation as a reasonable trade-off between battery life and usable capacity.

The more a lead-acid battery is discharged, the more some undesirable chemical reactions take place. As the solution weakens in strength, the sulfur being deposited onto the lead plates tends to form a harder, more crystalline form of lead sulfate. This harder form of lead sulfate tends not to re-dissolve the sulfur when a charging voltage is applied. Where the hard sulfate is formed on the plates is now unusable capacity (called sulfation).

The lower the sulfuric acid strength, the higher the proportion of hard sulfate on the lead plates - and it's not linear. Discharging a battery down to 11 volts gives you maybe 10 recharge cycles before capacity is a fraction of what it was - and the figure is maybe 2 full discharge cycles for a starting battery.

Battery manufacturers have charts showing number recharge cycles vs SOC, and often publish the cycles at 50% SOC. IIRC, a Sears Marine Diehard has about 400 cycles. My GC-2 Interstates (from Costco) are rated at 800 cycles at 50% SOC. If you never go below 70% SOC, you add to the cycles, but have a lot less usable capacity.

Yes, you need to recharge when battery voltage reaches 12.0 to get a reasonable life out of your batteries.

Hope this helps.
Fred W
2019 Flagstaff T21TBHW A-frame
2 GC-2 210AH 6V batteries to carry us for 4 nights dry camping at 5hrs/night heater run time