Splitting auxiliary battery banks

We see this idea all the time. Unless there are very strong reasons for doing it, it really is extremely bad practice. However only the final owner/operator of the installation can decide what is or is not a very good reason for doing so.

A valid reason for splitting a single auxiliary battery bank into two or more battery banks would be in a life support system. In this instance it is, of course, very important that the life support equipment continues to operate even after the batteries that power other less important loads have been totally discharged. And really, this is about the only valid reason for doing so.

Now I suppose the definition of "life support" varies from person to person.

For instance someone may consider a certain item of medical equipment to be life support. Another may consider a fridge (for beer) to be life support. Yet another may consider a TV to be life support. In certain installations lighting itself really could be a life support system.

Another valid reason might be communications equipment. For instance in an ambulance, or remote residence in a freezing mountain range. In these instances, communications equipment could be vital to life.

Whatever, the important point is that it should only be done if it is considered absolutely critical to maintain power to a certain load (or loads) when other battery banks have been discharged.

The reason is, yet again, Peukert. But only after some careful thought of how a typical system is used does the actual reason become apparent. A cursory glance and calculation (even taking Peukert's effect into consideration) seems to produce no difference in available battery capacity. So let's start there.......

Assume a single battery bank of 400 amp hours. With an average load on it of 30 amps. Let's say Peukert's exponent for these batteries is 1.3, the typical figure for deep cycle wet cells.

This gives a total run time, to 50% state of charge, of just under 6 hours (use the Peukert Calculator).

Now split this battery bank into two banks of 200 amp hours each. With a load of 15 amps on each battery bank.

Again use the Peukert calculator, you will see that the available run time for a 200 amp hour battery bank, at 15 amps discharge is exactly the same as 30 amps from a 400 amp hour battery bank. So the total available run time remains identical.

The problem is that this is not how typical battery banks get used. Instead, the loads tend to be intermittent and alternate. For instance the fridge may have a 1 in 3 cycle. Meaning it runs intermittenly for 8 hours out of every 24. Lighting is also used in a similar way. It isn't on all the time. Most other loads are similar. Inverters, TVs,

microwave ovens etc.

So in order to clarify this situation, let's take the typical reason we see on boats for splitting the battery bank into two. It is nearly always to have a separate battery bank for the fridge.

The following shows why this is such bad practice.

Let's take a typical 12 volt fridge. It will use around 40 amp hours per day (obviously this depends upon the ventilation, the amount of food or beer in the fridge, how often the door is opened etc etc etc but we have to start somewhere). It will do this by being off for about 16 hours a day and on for about 8 hours a day. During the time it is on, it will draw around 5 amps. If you calculate this you will see it comes out at 5 amps for 8 hours = 40 amp hours. This means an average of 1.67 amps. But the actual current draw is 5 amps for a third of the time.

If we now use the actual current draw of 5 amps for a third of the time we get a run time of 121 hours to totally flat, so this is 60.5 hours to 50% state of charge, but this is only for a third of the time so the true run time will be 3 times this = 181 hours. This is somehwat less than the calculated figure using the average current consumption. This is to be expected as a direct result of Peukert's effect (drawing 5 amps from a battery removes more power from the battery than drawing 1.67 amps for three times as long - this is what Peukert's effect is all about).

However if we now split the battery bank into two 200 amp hour battery banks we find that 5 amps for a third of the time gives us 49 hours to flat, which is 24.5 hours to 50% state of charge, but again this is for a third of the time so 3 times this is 73.9 hours.

In the first example, calculated correctly using the actual current draw, as opposed to the average current draw we got 181 hours run time from the 400 amp hour battery bank.

In the second example, using the same calculation we got 74 hours run time, for the same equipment, from the 200 amp hour battery bank.

So if the battery bank was split in two, then each bank used for the fridge, and fridge alone, we would get a total run time of 148 hours. Whereas from the full 400 amp hour battery bank we would have got 181 hours.

And all intermittend loads will work the same way.

In effect, splitting the battery bank into smaller banks gives a smaller total available battery capacity and therefore a shorter run time.

All due to Peukert's effect. But the effect doesn't become apparent in calculations unless the actual current draw, calculated on a cyclic basis, is used for each item of equipment as opposed to the average current draw of all the loads combined.

Also consider that the very fact that the battery banks have been separated means that each battery bank will, in all probability, be discharged to a lower state of charge. This shortens the life of the batteries (see the 50% rule).

And finally there is a real sting in the tail. This practice of separating the battery banks is usually done to ensure one particular piece of equipment (often a fridge) maintains it's battery supply after the main battery bank has been flattened.

This implies 2 things. Firstly, that the installation is regularly discharging the batteries to such a low level that the fridge will not operate. This of course implies very deep discharges which is very bad for the life of the batteries.

Secondly, separating the battery banks will even further accelerate this problem as the main auxiliary bank is now even smaller than it was before it was split in two.

The correct solution is not to separate the battery banks. It is to increase the size of the total battery bank or to ensure that the battery bank is being correctly, and fully, recharged.

And finally, having typed the above I would like to add one last thing.....

There exists a small number of people who carry out this practice of separating the auxiliary bank into two or more battery banks (without good reason to do so) and who "know" they are doing the best thing. Even though it is so easy to show otherwise, either by theory, maths, or actually comparing a single auxiliary bank to 2 separate ones and measuring the run times and battery life. They just "know" their way is the right way. Even though they are wrong.

I am under no illusions that their way of thinking can ever be changed. But I have at least shown mathematically why they are incorrect. Which is about as much as I can do.


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