What size battery charger or alternator do I need ? I hear so many different things I don't really know what size to buy ?

This question crops up often. Salesmen usually reply with something along the lines of "10 percent of the battery capacity" or "15 percent of the battery capacity". An engineer will either ask you a LOT of questions about your installation and useage requirements or explain to you the effects of charging at different rates then let you make the decision.

The fact is that the size of charger required depends upon too many variables for there to exist a simple rule of thumb such as "10 percent of the battery capacity".

I offer the following by way of explanation:-

Let's look at 2 extremes of battery charger size in relation to battery capacity and see what we get........

In both these cases we will be considering a wet cell deep cycle battery and a real 3 stage smart charger. It doesn't matter whether that is an AC mains powered charger or an alternator with a true 3 stage controller. For our purposes here it makes no difference.

The first thing to remember is that batteries are "consumable" items. They do not last forever. Each time a battery is charged or discharged it shortens it's life. The harder it is charged or discharged the more the life is shortened. There exists a certain folklore that "batteries need to be worked". It is total rubbish. The more a battery is used the shorter it's life will be. The less it is used, the longer it's life will be.

The only exception to this rule is that batteries should NOT be regularly discharged by only a few percent. Discharging a battery by less than 5% can shorten it's life and limit it's ability to deliver high currents. This is due to the way the sulphate builds up on the plates which is very irregular during the first stage of the discharge cycle. Either don't discharge the battery at all, or ensure it is always discharged by more than 5%. This is not a particularly well known phenomenon (except by the battery manufacturers) and could be the genesis of the urban folklore/myth of  "the batteries need to be worked".

Example 1   A 100 Ah battery with a 1 amp charger.

Assuming the battery has been discharged by 50% we have to replace 50 Ahrs. On the face it this will take 50 hours but it's not that simple. Generally (all other things being equal) we need to add 40% to this figure (this is because, as the battery becomes more fully charged it will accept less and less charge current) so this will give us a recharge time of 70 hours. Now in some installations this may be acceptable. In others (certainly on my liveaboard) it would be totally out of the question.

2 points to note here.

1. This battery would have a good long life. It is being "gently" treated.

2. Because the battery was very slowly charged it would have a genuine, good charge. It really would be 100% charged (see below for counter example and explanation).

Example 2   A 100 Ah battery with a 100 amp charger.

Again assuming the battery has been discharged to 50% we have to replace 50 Ahrs. For the same reasoning as given above this will take 42 minutes. "Wonderful" everyone thinks, such a quick recharge time.

There are 2 serious problems with this fast recharge cycle. Firstly, due to what is known as "surface charge" this battery will not actually be fully charged. The charge rate would have reduced considerably during first 15 minutes or so. This is because at such a high (and therefore fast) charge rate the charge (which remember is actually a chemical reaction between the acid and the lead plates) would not have been able to penetrate deep into the lead plates. The only parts fully charged would be on the surface of the plates, hence the term "surface charge". Once the charger was disconnected there would be some balancing of this charge due to internal chemical reactions within the battery resulting in the surfaces of the plates and deep within the plates becoming equally charged, at considerably less than a fully charged state.

Secondly and perhaps more importantly, this battery was very heavily charged, this causes severe stresses within the battery which SERIOUSLY shorten it's life.

Somewhere in between these two extreme examples is the typical battery/charger combination.

You can clearly see that a small charger (in relation to the battery capacity) has the benefits that it.....

A) Treats the battery "gently" thereby resulting in a long battery life.

B) Ensures the battery is indeed fully charged and not simply "surface charged"

It has the disadvantage that it takes an awful long time to recharge the batteries.

A large charger (in relation to the battery capacity) has the benefit that it.....

A) Recharges the batteries quickly.

It has the disadvantages that it......

A) Doesn't ensure a genuine 100% charge due to surface charge effects.

B) Severely shortens the life of the battery.

Finally, let's assume someone did actually fit a charger of, say, "10 percent of the battery capacity" because a salesman told him that was what he needed. This owner has a 200 amp hour battery bank. He now has a 20 amp charger. What about the fact that he has almost permanent loads of 25 amps on the batteries? The charger will not even keep up with the demand of the loads let alone charge the batteries.

From the above I hope it is very clear that for someone to state "you need a charger with 10% (or some other figure apparently plucked out of thin air) of the capacity of the battery bank" simply shows a total lack of understanding of battery charging and DC power systems.

The fact is you have to weigh up various different aspects such as required charge time, expected battery life, insistence on obtaining a full charge, average loads during charging etc etc etc.

There are no hard and fast rules on finding a suitable compromise but experience does indicate the following (very) general results.....

Users with a charger of 10% or less of the battery capacity generally complain of excessively long recharge times. Their batteries do however last a long time.

We have experienced many users with battery chargers of around 100 to 150% of the battery capacity. They gloat at their quick recharge times. They also continually complain that "batteries these days are rubbish and don't last 10 minutes"

Users with chargers of around 25% to 30% of the battery capacity tend to seem reasonably happy with the compromise they have of battery life versus recharge time.

Other users (myself included) tend to go slightly higher than this and generally run a charger between 35% to 50% of the battery capacity, enjoy the slightly shorter recharge times, but accept the (again) slightly shorter battery life.

Deciding on the required charger size is a series of compromises. Until there is a drastic breakthrough in battery technology this will remain the case.

Draw your own conclusions.


During the last few years AGM (Absorbed Glass Matt, a type of VRLA [Valve Regulated Lead Acid]) batteries have become freely available outside the rather expensive markets they were previously seen in. The same rules apply as above but in the case of these batteries much higher charge rates can be tolerated and sustained. Note that this is the charge current. Excessive charge voltage will damage these batteries very quickly indeed. They are very intolerant of high charge voltages (with the exception of Optima Spiral AGM batteries which can tolerate higher charge voltages).

And as we've said elsewhere, like most things, battery technology changes.

There are now two distinct type of VRLA batteries. In one type the only difference is that the electrolyte is held in a glass matt. This type usually use charging voltage similar to standard flooded wet cell batteries.

The other type has additional chemicals (the same as Gel cell batteries) that affect how the batteries perform and in particular the terminal voltage. It is typically around 0.2 volts higher than an equivalent flooded battery. This type often requires a lower charge voltage than the first type of VRLA mentioned above.

Check the battery manual for the correct charge voltages!


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