Monday, 22 August 2011

Battery Charging

As we plan to have a large (and expensive) domestic battery bank I’ve been reading about charging lead acid batteries. No equipment lasts forever, but I do want to protect our investment. The first thing I’ve identified is batteries can be damaged if they are discharged more than 50%. Next, it appears recharging can be quite complex. I’m going to explain my understanding of the problem and solution using an analogy. Readers with more experience are welcome to correct me by leaving a comment.

Consider the battery bank to be a water tank. Water (electricity) is drawn off and the tank gradually empties. The tank shouldn’t be emptied more than 50%.

When the tank has to be refilled the water can either be trickled in or poured in under pressure. Obviously the more pressure and volume of water being poured into the tank then the quicker it will fill (with electricity, volts = pressure and amps = quantity). However if there is too much pressure or too large a quantity of water the tank might burst. In a battery the electrolyte can boil, the plates warp or the case split. So too much, too quickly, can be a bad thing. Too little may result in the process taking too long or the tank not being filled to maximum capacity.

However the water tank isn’t an empty tank. It’s actually a tank filled with a sponge. In the initial stages of refilling the tank the sponge readily and quickly absorbs the water. Eventually it will reach a saturation point where it doesn’t matter how much pressure and volume of water you attempt to force into the tank it simply can’t be absorb in that volume or pressure. Nevertheless there are pockets of air in the sponge, so the tank still hasn’t been filled. But if we are prepared to be patient and trickle a small amount of water into the tank it will all gradually be absorbed until the sponge is completely saturated. If we don’t completely fill the sponge in the tank then those dry pockets will eventually become permanently unavailable and the total capacity of the tank will be reduced. This is a cumulative process until the tank is essentially worthless.

So our battery bank is going to need a large amount of electricity to recharge the batteries at the beginning of the process and then a “trickle” charge to complete the final top up. Running the engine and alternators during this latter stage is a waste of energy (diesel). For the top up stage we need to consider either using shore power or perhaps solar panels.  Alternatively, we can accept the battery life will be reduced through failing to fully recharge the batteries.


Peter and Margaret said...

Hi Tom, Here are my views for what they are worth. I did my own blog on energy requirements here: Generally, a quick charge is less preferential to a long slow one, and the quick one can cause the electrolyte to boil. KL is fitted with one of these: which drastically increases the efficiency of the alternators, but also increases the risk of fluid loss during the process, which is explained in the instruction manual. When we are moored longer than one night, I use a small portable generator, if I can, without annoying others, to operate my multi-stage battery charger, which is much kinder to the batteries, and its use ensures their maximum life expectancy.

Jannock said...

Your analagy has one thing missing - the small water leak from the tank that is the natural charge degradation when the batteries are left untouched.
We have a small 30W solar panel on Jannock's roof which, during fine weather, maintains the charge state of our 330A/H domestic bank when we are not using her.
It is noticeable during the winter months that a larger panel is needed to maintain their charge whilst the boat is closed up but we have a shoreline on the mooring that can be used there.

Bruce in Sanity said...

I really like your analogy! It works for discharge, too. A rapid discharge doesn't use all the available charge, because the water can't percolate through the sponge fast enough.

Traction batteries can be discharged below 50% as I understand it, in some cases as low as 20%, but they do need the regular overnight recharging to recover properly. PB batteries reckon at least every two weeks.

It's a dark art, battery specification, like props!



Nick norman said...

Nice analogy! My only comment is that the 50% figure for discharge is not a magic number, just a ballpark number. The curve of battery life against discharge depth is fairly straight around 50%, so there is not that much life difference between say 45% and say 55%. It's not like the graph line falls off a cliff at that point.