Tecktalk - Winter Battery Storage

Telltale Article - by George de Witte.

The topic of course is whether to haul the batteries home from your boat to store them in a kinder environment or just leave them on board during our harsh winters. I was quizzed about this subject a few weeks ago while sipping a cool one on the NSC deck and admiring another glorious late summer sunset. In this article an attempt will be made to clarify some of the technical issues re winter storage and you can take it from there. In my research to prepare for this article it was clear that there are as many proponents as opponents to battery removal for the winter and I suspect the same division of opinion prevails around our membership.

Some Basic Battery Knowledge

The standard wet cell lead acid battery works as follows: each cell consists of two plates suspended in an electrolyte solution of sulfuric acid. One plate is coated with an active lead material. When a battery delivers electricity, the sulfate from the electrolyte combines with the active lead and forms a coating of lead sulfate on the plate. When most of the sulfate is depleted from the electrolyte, the battery is said to be “dead”. Fortunately the process is reversible and when current is forced into the battery, the sulfate from the plate is returned to the electrolyte, so the battery can supply electricity again. Each cell delivers about 2 Volt, so a 12 Volt battery therefore contains 6 cells.

When the battery gets charged and the sulfate is forced back into the electrolyte, its density increases. In more technical terms its specific gravity (SG) increases and this can be used to measure the state of charge of a battery. A typical SG table at room temperature looks as follow

Charge SG Voltage
100% 1.265-1.275 12.6
75% 1.225-1.235 12.4
50% 1.190-1.200 12.2
25% 1.155-1.165 12.0
0% 1.120-1.130 11.7

SG is hard and a little messy to measure. Hence a Voltage column is added to the above table as well.  With the advent of cheap and accurate Digital Volt Meters, battery voltage is now easy to measure. The only requirement to use voltage measurement as a charge indicator is that there be no load and the battery has settled for a while.  (Preferably a few hours)

Batteries get “old” when the sulfate on the plate hardens and therefore the sulfate no longer dissolves in the electrolyte during charging to produce electricity. In practice this means that the battery no longer delivers its specified Amp-hour capacity. This process is called sulfating and can happen when the battery sits unused for a long period or when a deep cycle battery never really gets fully charged due to frequent use. When a battery is sulfated, it can sometimes be restored with a process call equalizing. This involves charging at a higher voltage than the normal float voltage of 13.2V and frequent measuring of the SG. In other words better left to an expert.

From the above it should be clear why there is such a discrepancy in marine battery life expectancy. It can range for 2 seasons for an abused battery and in excess of 5 seasons for a well maintained one.

Batteries and winter

With this bit of battery knowledge it should be clear that in a perfect world the batteries should be taken home during winter and kept in a warm place.  Once a month drain them to about 50% charge and then recharge them. At least that is the advice from one of the leading marine battery manufacturers.

The flip side is that most boat owners ignore this advice and for obvious reasons. In larger boats, batteries can weigh upwards of 100 lb. Lifting these monsters 10 ft down a ladder is a chore to be avoided if at all possible. The reality is that good to high quality batteries can provide service lives of 5 years or more if precautions are taken to ensure that the batteries don’t freeze up during the winter. Batteries don’t freeze at temperatures down to -40 C if the charge level is kept above 75%. However if they do freeze, then irreparable damage occurs and they need to be replaced.

So what to do if you decide to keep the battery onboard like most boat owners do. Begin by charging the batteries to 100% as late as possible in the fall. Then disconnect the ground lead on each battery (Black wire on old and yellow on new boats) This prevents a trickle discharge due to unknown loads that are still connected even if you think that all loads are turned off. For instance on my boat there is a leakage through the charger of 80 milliAmp. The charger is permanently connected to the batteries, so the charger alone would take the battery charge down to 70% within a month.

The other issue at play is internal discharge. A healthy battery discharges about 25% in 30 days at 27 C, but this increases to 100 days at 10 C. So in our climate a battery should hold its charge from say mid November to mid April. So start by cleaning the top of the battery so there is no discharge path between the terminals as a result of years of accumulated dirt.

Next I suggest if you want to play it safe, measure the battery charge status sometime midwinter. If the no load voltage is anywhere near 12.5 volt, then there must be a higher than normal internal discharge and you have to recharge the battery. If the voltage is 12.6 or more, you are fine and can relax for the rest of the winter.

Midwinter Charging at NSC

There are solar power trickle chargers on the market. I have no experience with them, but if you buy one make sure it comes with a smart regulator.

The other option of course is to run an extension cord to the nearest power pedestal in the yard. You should know that current club policy is to turn off the power in the yard during the winter months. I have discussed this with the harbour master prior to writing this article and he has indicated that for the right reasons he will temporarily restore power in the yard for battery charging only. This will require access to the circuit breakers in the green boxes in the yard. If  there is snow and ice these boxes will not be accessible, so assume no power for at least January and February, perhaps into March if it’s a long winter.

Another question I was asked is how to use a standard 100ft household extension cord to connect the yard outlet with the 30 Amp shore power plug installed on most boats. Marinco makes a short adapter tail with a 30 Amp 120 V 3 wire 2 pole twist connector on one end and a standard household plug on the other end. It costs $50+ which seems like a lot. Can you make one yourself is the question. Well I have priced the 30 Amp /120V/2pole/2wire connector in a few stores in the range of $21 to $40. So if you can find the $21 one, also buy a cheap power bar and cut off the power bar itself. Then make the following connections: green(ground) wire to the dark or green painted screw, black(hot) wire to the brass coloured screw and the white(neutral) wire to the silver coloured screw. Make sure the stress relieve is stress relieving and with silicone or electrical tape try to make the whole thing waterproof.

Hopefully this answered the questions of one of our member bar flies. Feel free to call or email if you have any constructive comments or questions.