Battery testing and replacing

We are still in Florida. Not in the Bahamas. While waiting for our waylaid UPS packages – which have now largely arrived – my husband (Rick) decided to delve into our struggling battery situation.

We are on “the hook” – bobbing around on our anchor. At a marina you can plug into shore power. But out on the water we are reliant on solar and battery power. We have had trouble maintaining adequate power without running the generator every day.

At first we questioned our energy consumption and then the battery charging regime. But, that didn’t seem to be the issue. In fact, with four children – a generous brood by today’s standards – and dying cell phones, I began to wonder if we were living like the Amish.

My husband went to work, investigating our batteries. Dave got involved. Rewiring was done to effect an improvement he suggested (which doubled our capacity – see (To load balance take your positive and negative leads from diagonally opposite corners of your battery bank). But still our batteries were not performing as they should.

We should be able to anchor out for at least two or three days on battery and solar power, without running the generator. Our batteries are only two years old. The batteries were purchased in Cartagena, Columbia. They are a Bosch P5 brand that is unknown in the US and that appear to have evaded the Internet. To say that Bosch has been unhelpful in our attempts to obtain the specifications for the batteries would be giving them too much credit.

We suspect our Bosches are unfortunately regular car starting (cranking) batteries, which are generally unsuitable for marine applications. Car batteries are designed to provide a lot of current for a short period of time and they don’t like being discharged much past 85%. Marine batteries on the other hand, need to provide less current but over longer periods of time and they need to survive deeper discharges. Using a car battery in a house bank on a boat can kill it in a matter of months. How we got ten months into our refit to be tripping over this issue now is beyond me.

More tests were performed. Two of our nine batteries seemed to be faulty. Calls were made. Four of our nine batteries were removed, then taken by dinghy, then by car to Miami (thanks to our friend Carl) for further testing.

The batteries came back clean – no faults found – all four passing both load testing and Midtronics electronic testing. The final comment from the service tech was that we needed to look elsewhere in the electrical system for our problem.

This was good news, in a sense. But that left us wondering why we are so short on power. My husband is a problem solver. He may have drawn on his background in space physics to see this problem through. I overheard him saying that batteries were very important to the space program. The lifespan of most satellites is governed by the lifespan of their batteries and their onboard fuel for maneuvering.

He was not convinced by the testing. Starting with a fully-charged battery, my husband decided to drain a single battery, and took measurements every 15 minutes.  He drained the battery using Christmas lights, soldering irons and some lamps borrowed from Dave. All together they yielded him a consistent 17 amp load, at least that’s what he said.

The battery took minutes, not hours to drip below 12.0 volts, leading Rick to conclude that the battery tests were superficial. I believe there was some mumbling about surface charge versus absorbed charge. But, in short, we proved our batteries were pretty much toast. At that point we knew we had to buy new batteries.

You must replace all batteries in a bank at once. So, after swallowing this expense – our beautiful new Victron batteries will arrive this week. Thanks to Peter Kennedy at for helping us with the battery selection. In geek speak – my husband is quite happy that we managed to add new heavy-duty, deep-cycle AGMs for roughly $2.17 per amp hour, delivery included.

After the new batteries are installed we’ll wait for a weather window and sail to the Bahamas.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s