Sears Diehard battery update

You might remember our debate about which batteries to buy and our ultimate decision to buy 600 amps of Sears Diehard Platinum batteries (500 house plus 100 starter) which are repackaged Odyssey batteries (and the installation of that mass in our wee Pretorien). I should mention again that not all Sears batteries are Odysseys, the Diehard Platinum marine batteries are - I called Odyssey.

One major reason we chose them was that they can accept higher charge rates closer to 100% than other batteries. We had cruised on shorter trips for 3 years with only a 150 amp hour battery bank and long drawn out engine charging. That painful experience was a significant motivator in our desire to reduce the frequency and length of our charging sessions. In addition, if we can use our battery bank from 50% to nearly 100%, instead of only from 50% to 80%, the same battery bank will effectively be larger. How did that work out for us?

Well, honestly, the first answer is that we didn't really get to test out that theory very often because our solar panels worked so well. It wasn't until the last week or so of September that we had enough rain and clouds to need our Honda 2000 generator. This makes me wonder if high charge acceptance beyond 80% will ever really be a benefit for us because even though I expect our amp usage to increase as we head South, I also expect our solar panels to perform better.

The second answer is that during those couple of generator sessions we were struggling with how to get our charger to continue giving higher charge to the batteries.


The charger is a 3-stage charger and during bulk charging, it increases the incoming amps until it forces the voltage of the batteries to a certain level and then it adjusts the incoming amps (slowly declining) in the absorption phase while it maintains that voltage (chart from Magnum manual). The key here is that it is the target voltage of the battery in each case that is used to decide the incoming charging amps.

So if you have Odyssey batteries and want to follow their charging specs you set that voltage at 14.7 (chart from the Odyssey tech book). If we follow their charging procedure, we set 14.7 and the amps are adjusted to keep the batteries at 14.7. We can trick the charger into increasing the absorption charge time by telling it we have a larger battery bank than we do, but this just means that it will charge at whatever low amperage it needs to maintain 14.7 volts...not behaving like the thirsty camel we believed they could be. Nigel Calder was doing some tests showing that they could take a higher voltage but there is no way we are going to increase the voltage and risk a battery deformation or explosion until we hear some firm statements from the manufacturer about that.

The other reasons we chose them are:
- maintenance free (all AGMs - check)
- can be installed on their side (all AGMs - check)
- longevity (purportedly great quality - to be determined)

For now, everything works and any installed gear that works and doesn't need to be maintained is a winner. However, we will have to give them a few years before we can say if they were the right choice.