Battery Theory

[nolandinsight]

Do I understand this correctly?

If I have a bank of two batteries, both 12v 100 amp hour, wired in parallel, the bank will supply 12 volts at 200 amp hours capacity.

If I have another identical bank wired with the first to a 4 position selector switch, in the 'all' position the supply is 12 volts at 400 amp hours of capacity.

Is this right?

[donradcliffe]

yes, you have it right.

[Sailmonkey]

but to throw a wrench at you, to keep the batteries happy you should only really use 50% of the total AH capacity.

[scoobert]

with 6 batteries you have about 3KWH of usable power

[nolandinsight]

Right, so in the above scenario it's 200 amp hours usable, and that's to preserve battery life.

Scoob, you made my head sad. I've no understanding of kilowatt hours.

[Capt RonB]

If I interpret his math, he's saying that 200 amp hours x 3 pairs of batteries is 600 amp hours. At 12v, 600 amp hours - 12v X 600ah = 7200 watt hours, half of that is 3600 watt hours or 3.6 thousand watt hours hence 3 something KWH?

With two sets of batteries that total 400 amp hours and assuming that you only use them to 50% of capacity before charging, you should treat them as a single bank. That is keep the battery switch on all ... all the time. Ideally your engine starting battery is separate from this 'house bank'. The reason you want to use them as a single bank has to do with Peukert's Law and discharge rates. Basically 400 amp hours of battery will discharge slower than two 200 amp hour batteries. Without going through the math, it probably goes something like this; if one 200 amp hour battery discharges to 50% of capacity in 4 hours and therefore two would last 8 hours then the 400 amp bank might make it 8.6 hours assuming the same load.

"Peukert's law describes an exponential relationship between the discharge current (normalized to some base rated current) and delivered capacity (normalized to the rated capacity), over some specified range of discharge currents." (Wikipedia) Peukert constants vary depending on battery type too.

From Wikipedia here's a spreadsheet that illustrates the effect http://www.smartgauge.co.uk/calcs/peukert.xls

[noelex 77]

Originally Posted by nolandinsight

Right, so in the above scenario it's 200 amp hours usable, and that's to preserve battery life.

Scoob, you made my head sad. I've no understanding of kilowatt hours.

Correct assuming the batteries are
100% charged
Have their new capacity
The discharge rate is close to that specified when the capacity is measured.
You stop at 50%

The last three are reasonable assumptions, but it is difficult and very slow to charge a lead acid battery to 100% unless on shore power, or you have a large solar array
If charging is stopped at 80% (which is often done when charging with main engine, or generator) you only have 120AHrs of capacity (to 50%)

To convert AHrs to KwHrs just multiply by the battery voltage, but AHrs is a better unit to use, on boats, for several reasons the main one is the difference in charging and discharging voltage makes some allowance for battery ineffiency.

[skipmac]

Originally Posted by noelex 77

Correct assuming the batteries are
100% charged
Have their new capacity
The discharge rate is close to that specified when the capacity is measured.
You stop at 50%

The last three are reasonable assumptions, but it is difficult and very slow to charge a lead acid battery to 100% unless on shore power, or you have a large solar array
If charging is stopped at 80% (which is often done when charging with main engine, or generator) you only have 120AHrs of capacity (to 50%)

To convert AHrs to KwHrs just multiply by the battery voltage, but AHrs is a better unit to use, on boats, for several reasons the main one is the difference in charging and discharging voltage makes some allowance for battery ineffiency.

Pretty much covers the question, accurately and in detail.

[smac999]

are you asking because you are trying to hook up / set up a battery monitor?

if so, you will have issues with the split banks. unless you have 2 monitors

[pbiJim]

All of this assumes that the battery switch is wired correctly. I have seen several that were not.

Also, some industrial deep cycle batteries (like the ones used on electric forklifts) are rated to run as low a 20% charge before being recharged. The quality of batteries varies greatly. Deep cycle batteries are very different internally compared to regular car batteries. A regular car battery has the geometry of it's internal plates configured to give increased cold cranking amps at the expense of reserve capacity & low charge tolerance.