First, there are many good postings on this topic in the archives.

"Dual" 12 volt batteries configured as a 24 volt battery should be treated as a single battery. They should be purchased, installed and REPLACED at the same time, even if one is still "good" (see below).

Monitoring the intermediate voltage (between the batteries) won't tell you anything that monitoring the buss voltage can't. The problem is resolution. Half a volt is a lot to a cell but not a lot to the buss. However, a comparator that reports the difference between the two batteries in hundredths of a volt may give some advance warning of a cell problem. It is probably better to spend your money and time replacing the batteries more frequently.

I don't get the dual dual big battery configuration (four 20AH batteries configured in two 24V pairs). In a dual alternator configuration the second alternator is your backup and the electrons stored in the battery are the backup to the backup (tertiary backup). Having a second big battery means you have a backup to your backup's backup and that last backup weighs 30 pounds. I would rather have 5 gallons of fuel.

In my plane (28V) I have dual 20AH 12V batteries connected to a 60 A alternator (1680 watts) and a secondary alternator connected to a 2 AH (yes two) standby battery. The standby battery stabilizes the standby alternator, feeds the dome lights when the master is off and can be cross tied to the main buss. My plane is all electric and has plenty of power available to service all systems. I do not have air conditioning. Remember that a 60A 28 V alternator has more power output that a 100A 14V (1,400 watts) alternator.

Scott's series / parallel switch is interesting but it would only be useful in an open cell failure coupled with an alternator failure. The added complexity, weight and risks associated with the switch seem unwarranted. The risks include a switch failure that shorts the battery and accidentally actuating the switch causing the two 12 v batteries in parallel to be charged at 28 volts. Also, open cell failures are rare in RG batteries (less rare in wet cell batteries).

I appreciate that some aircraft need to relocate the batterie(s) to the tail for W&B but this is expensive in a number of ways. All the really high currents are between the battery, alternator and starter so these items should be a close as practical. Large diameter copper wire is heavy and has internal resistance. The alternator is (typically) the primary source of electrical noise and the battery provides most of the noise damping. A good, low impedance circuit between these two is an important element in a "quiet" electrical system.  Starters require high peak currents. A good, low impedance circuit between starter and battery is an important element for easy hot starts. Do you see the trend?

For tail mounted batteries to have the same performance as firewall mounted batteries the installed system weight of the tail mounted battery must be higher.

My IV-P has an empty weight of 2,188 Lbs. Controlling the weight during the building process requires scrutinizing every ounce. Watch the ounces and the pounds take care of themselves. When building airplanes, there are definite merits to asceticism.

Regards
Brent Regan

>From a previous post on batteries:

<<When you discharge a Lead-Acid battery the lead oxide on the positive plate and the lead on the negative plate combine with the sulfuric acid in the electrolyte to produce lead oxide on both plates, water and free electrons (a lot of them). When a battery is fully discharged the positive and negative plates are chemically identical (lead sulfate) and the battery can then, theoretically, be charged in with the opposite polarity.

Lead, lead sulfate and lead oxide all have different densities so that during the charge and discharge cycles the plates are expanding and contracting slightly. One of the limiting factors in battery life is mechanical fatigue of the plates. Over time they disintegrate and , in wet cells, fall to the bottom and short the cell.

Batteries designed for starting engines have a low cell resistance and a high peak current capacity. Batteries designed to provide standby power are optimized for maximum power density and are referred to as "Deep Cycle" type, as they are designed to be discharged until their cell voltage falls to a fraction of the initial voltage.

A typical (in this case) battery is composed of cells. Each cell produces a nominal 2.0 volts so several cells are connected in series to produce the needed voltage. Because they are manufactured at the same time, these cells are nearly identical in performance, which is a good thing because if one cell fails the battery fails. A battery only has the capacity of the weakest cell. Because both charging and discharging produce heat and the cells are stacked side by side, the middle cells run a little hotter than the cells on the end and therefore end up having slightly different performance. This difference is small and insignificant MOST of the time. The time it becomes significant is when the battery is completely discharged. During a complete discharge, one cell reaches exhaustion first but it is still part of the circuit so the other cells are still trying to push electrons through it. What happens when you push electrons through a battery rather than it doing the pushing? It charges, but it charges in the reverse polarity (remember the plates of a discharged battery are chemically identical). This "cell reversal" is very damaging because a large portion of the remaining charge in the stronger cells is dumped into the weakest cell. The mechanical stresses on a "reversed" cell are very high due to the density changes mentioned above. To make matters worse the internal cell resistance increases as the battery discharges so the batteries ability to accept a charge current also decreases. This is why the charging current on a completely dead battery starts out low then climbs rapidly and then falls slowly as the battery charges.

Battery life is frequently specified in charge discharge cycles and a cycle is usually specified as a percentage of capacity (e.g. 80%). Using only a portion of the capacity will extend the life and using ALL the capacity will shorten the life. Running the battery all the way out to cell reversal will GREATLY shorten the life. How much is that? I can't say. But it would be a good idea to replace the badly abused battery at the next annual or major service.

The energy capacity of a battery decays with time and use. If you wait until you notice that your battery performance is marginal it will likely be the time you need it most. It is better to replace on "time" as a maintenance item than on "performance" as a repair item. Depending on service, temperature swings, and flight hours, replacing your battery every two to three years would be a good plan.

Regards,
Brent Regan
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