Monday, 11 May 2015

Battery wiring

I've had an interesting discussion with members of the CanalWorld forums about the wiring to Erin Mae's batteries. The basic question was why Chris Gibbons' SmartGauge website states so forcefully, but without explanation, that charging cables should always be connected to the same battery posts as the load.

It's a final comment on a page dedicated to explaining how to connect the load to four (or more) batteries so that they are balanced and age at an equal rate. His main concern is with the surprisingly shocking results you get when you connect +ve and –ve leads to the same end of a battery bank. He accepts that, for most situations, simply taking them from opposite ends of the bank gives a result which is tolerable. But then he shows how to wire them so that they are completely balanced.


He credits "smileypete" from the forums for this arrangement (and Pete was one of the contributors to the discussion I recently initiated). It shows the supply cables attached to the central batteries in the bank. The main feature is that each battery is connected to the load through one shorter and one longer cable.

Now I had noticed that, on Erin Mae, the cable to the 12 volt distribution panel is taken via an isolator switch from one end of the bank, while the lead to the inverter to give me my 240 volt circuit is taken via its own isolator switch from the other end of the bank. I had decided, on the basis of the SmartGauge page, that this was probably not a Good Thing. Deciding what to re-wire to where, however, involved thinking about where the charging cables were attached – and that produced my question for the CanalWorld forum. Why was Chris so emphatic about the charging leads going to the same battery posts as the load leads?

One of the difficulties that I could see was related to stretching the existing charging cables to the posts indicated on Chris's diagram. I was puzzling over this just before going to sleep on Saturday night, and suddenly realised I could rearrange the cables so that the leads went to the end batteries, not the central two. I leapt out of bed and sketched it on paper, and on Sunday afternoon drew it up on my computer.

With delight I went to post it on the forum thread, only to discover that Pete had got there about half an hour before me, and posted his original diagram.


You can see this is effectively the same as mine, so why Chris changed it for the SmartGauge website, I don't know.

My conclusions from the forum discussion are:
  1. I don't want to move the inverter cable, since it is also a charging cable and is attached to the same end of the bank as the leads from the alternator and solar systems. No one offered any real defence for Chris's assertion about charge and load going to the same posts, but if I'm going to move something, there seems little point in it being this cable.
  2. I intend to move the connection from the 12v circuit isolator switch to the opposite end of the battery bank, where the inverter and charger leads are attached. This will involve buying a longer cable to stretch the length of the battery box.
  3. I need to get to grips with the negative cables, of which there seem to be a good number, work out where they all go, and ensure they are all attached to the correct battery post at the opposite end of the bank from the charging and load leads.
  4. If I am still alive, not having perished from electrocution, conflagration, too much work or old age, it would be nice to adapt the intra-battery wiring to correspond to Pete's scheme. That should entail getting two longer leads, and ensuring that there is room for all the connections on the posts. 
We're off Erin Mae for a couple of weeks, or else I would illustrate this with a couple of photos. I just hope that, when we return following next weekend, the reality matches my memory and that I am able to engage in some simple but effective re-wiring without blowing us all to bits.

(This post was edited because of difficulties uploading the diagrams. Hope it appears OK on your device.)

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