Summary of charging, split charging and battery monitoring.
We have to come up with a workable solution, to enable charging to both batteries.
As I said at the beginning, we're going to forget what has been done before, we're going to forget what other people do. We're going to look to the ideal solution. What would be perfect.
The reason for this is simple. It is that the systems many people are still installing were developed 20 or 30 years ago when they were the only choice. Nothing else was available at the time. However technology has moved on substantially since these systems were developed. Unfortunatley some people seem to be stuck in a time warp and are still installing 25 year old designs when far better solutions are now available.
We'll start with the engine driven alternator. i.e. a single charger.
Let's just stop for a moment to think about what we are actually trying to achieve here. We want to be able to charge both the separate batteries, but when we are not charging, we want to be able to discharge either battery, without discharging the other. So the lighting etc does not flatten the engine start battery, so that starting the engine does not discharge the other battery.
So what are the options?
We could fit 2 alternators. One charging each battery. This is obviously a warkable solution but it does have it's problems.....
A. There may not be enough room round the engine to fit another alternator.
B. If there is enough room to fit another alternator, there may not be sufficient pulleys to drive it. If that is the case then it may be necessary to use the existing pulleys and install a longer belt. This will reduced the "wrap around" on the pulleys which may cause them to slip.
C. Even if these problems are overcome there remains the issue that the alternator charging the engine battery is doing very little once it has recharged the engine battery (which will take a very short time - say 15 minutes at the most). This is wasted charge capability that really should be diverted to the other battery.
D. Fitting another alternator can be very difficult involving making up brackets, welding, grinding etc. Not really something most owners can achieve simply.
E. This "solution" does nothing to allow other chargers (say wind turbines, solar panels etc) to recharge both batteries.
F. Fitting another alternator gives the engine manufaturer another "get out" to avoid warranty claims. This especially applies in the case of vehicles. We do know of one case where the engine manufacturer claimed that the additional alternator had exceeded the side thrust rating of the front crank shaft bearing and therefore simply refused to honour the warranty on an almost brand new vehicle with a serious engine problem.
Another option would be to fit 2 switches. One between the alternator and battery 1, another between the alternator and battery 2. We can then switch the charge to either battery, or both of them. This of course will work very well provided the switches are sufficiently rated. But clearly there are several problems......
A. Both switches may be left switched on. This could result in lighting etc flattening the engine start battery. This is exactly what we are trying to avoid.
B. Both switches could be left switched off. This would result in neither battery being charged. As well as being left with flat batteries, this would also, in all probability, destroy the alternator.
C. The wrong switch may be switched on. This would result in the other battery not being charged.
D. The switches would, obviously, have to be quite heavy duty (in order to carry starter current) and this means that the main, heavy duty, battery cables have to be run into a position that enables the switch to be easily reached by the operator. This could mean long cable runs thus introducing large losses, plus the expense of the cable itself.
Clearly, whilst this idea could be made to work. It is far from ideal. Yet (just to ignore the rule about forgetting what has been done before) some installations use a very similar system, i.e. the "1, 2, both, off" rotary switch. Come on! This is the 21st Century!
I think you'll agree that this system is far from ideal.
So let's try another option. Again, we'll forget what others have done in the past.
What if someone made a "one way valve" for electricity? We could fit 2 of them, from the alternator to each battery. The power from the alternator could go through each "one way valve" to each battery, but discharging one battery could have no effect on the other battery because the "one way valve" would prevent the power going the wrong way through it.
Does anybody make such a "one way valve" ? Yes, they are called diodes. But they are far from ideal. They have so many inherent problems, that then require additional expensive equipment, in order to correct these problems (which, even then, only help alleviate a small number of the problems), that a better solution must be available. Have a look here for full details of why diodes simply will not do what we require.
If someone made a diode that didn't have a voltage drop across it, or that had a voltage drop that remained constant, irrespective of the current through it, then this would do what we require.
Unfortunately no-one makes one.
So we have to forget the idea of using "one way valves". They simply will not work.
Are there any options other than switches or "one way valves"? Any other possibilities?
Well if there are, let me know, because I can't think of any.
Page last updated 02/04/2008.
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