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bmc 1800 alternator

Adding to narrowboat battery bank

Michael Tyler

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Over the course of the last few weeks, I’ve been planning to upgrade the electrics of Bonnie Lady narrowboat.

One of the first jobs has been to add to the battery bank

Battery Bank

Is a collection of batteries which feed the boat and all the appliances on it.

Currently, we have 2 AGM 120ah leisure batteries in the bank. (There’s a starter battery, since none of the appliances run off this, I’ve not included it. It starts the engine. End of).

agm-leisure-battery-lp120
Due to the ‘live-on’ nature of the boat, this really isn’t enough:-

  • If you want to watch telly
  • If you want to be able to charge your computer, run printers, charge hoovers etc.
  • If you’re planning on having solar panels fitted

Solar panels will charge 4 leisure batteries during the course of a sunny day and less on more covered, cloudy conditions. The more batteries in the bank, the more cushion you have to soak up and store excess energy when it’s available. This may be through running the engine, or through the solar panels. When it’s raining, and you end up sitting around the boat all day with the lights on using the internet, you’ll really appreciate the difference!

Adding batteries to the battery bank

Major considerations are:-

  • Batteries need to be of the same type: You can’t mix AGM with lead batteries, or Gel with Lithium Ion. Better to stick to the same type.
  • Batteries need to be of the same Amp Hour: If you’ve got different AH thresholds, the different batteries are going to charge and discharge at different rates, not good for the lifespans of the batteries. Spread the load. Get all the same AH.
  • Batteries should be of the same age: If you’ve got some three year old AGM’s, and you’re putting in two new Gel batteries, this will place an additional stress on the new batteries. As they compensate for the wear and tear of the older batteries, they will wear out much more quickly until they become as aged as the originalvbatteries, but in half the time!
  • Loom arrangement: As the alternator charges them, and the boat wears them down, you need to get the correct battery bank wiring arrangement to ensure even charge and discharge across all the cells.

Upgrading the alternator

Modern batteries have modern requirements…… Old boats have old equipment, which may not be up to modern standards for charging modern cells.

bmc 1800 alternator
bmc 1800 alternator

The BMC 1,800 marine engine has an old Lucas alternator, which can put out as little as 10 amps.

The lower the ampage, the more like a ‘bench charge’ or ‘surface charge’ it is. It will charge your batteries short-term, over the longer term, it’s recommended that you put a greater, ‘deep’er charge onto the batteries.

If you’re using your batteries day, day out, you want to be putting through the maximum ampage possible, within limits.

I’m currently thinking an old alternator isn’t up to the job of charging the modern sealed cell batteries.

I’m going to get rid of it and replace it with this 55amp alternator from ebay.

Connecting new batteries in the battery bank

Which is the best way to connect new batteries in your battery bank?

Well, I came across this article in Smartgauge, it talks about all the battery arrangements, and which might be the best for your particular requirements.

Seeing as we’ve got two older batteries, the best arrangement would be

Arranging narrowboat batteries in series

The main feeds to the rest of the installation are now taken from diagonally opposite posts.

It is simple to achieve but the difference in the results are truly astounding for such a simple modification.

The connecting leads, in fact, everything else in the installation remains identical.

Also, it doesn’t matter which lead (positive or negative) is moved, Whichever is easiest is the correct one to move.

The results of this modification, when compared to the original diagram are shown below. Only that one single connection has been moved.

After this simple modification, with the same 100 amp load….

The bottom battery provides 26.7 amps of this.
The next battery up provides 23.2 amps.
The next battery up provides 23.2 amps.
The top battery provides 26.7 amps.

This is quite clearly a massive improvement over the first method. The batteries are much closer to being correctly balanced. However they are still not perfectly balanced.

How far is it necessary to go to get the matching equal?

Well, the better the quality of the batteries, the more important it becomes. The lower the internal resistance of the batteries, the more important it is to get them properly balanced.

So that now leaves the question of whether or not there is a wiring method to perfectly balance the batteries.

Before getting to that, it should be pointed out that doing the calculation is not actually required in order to arrive at the ultimate interconnection method. I simply did them to show the magnitude of the problem.

In order to get a better balancing it is simply necessary to get the number of interconnecting links as close as equal between each battery and the final loads.

In the first example the power from the bottom battery passed through no interconnecting links. The top battery passed through 6 links.

In the 2nd example (the much improved one), the power from the top and bottom battery both passed through a total of 3 links. That from the middle 2 batteries also both passed through 3 links which begs the question “why were they not therefore perfectly balanced?”. The answer is that some of the links have to pass more total current and this therefore increases the voltage drop along their length.

And now we get to the correctly wired version where all the batteries are perfectly balanced.

I’ll let you know how I get on once I’m able to take pictures again.

Here’s a short video of the harness.

Click here for reuse options!
Copyright 2017 Michael Tyler Sailor's Almanac: Further Narrowboat Adventures
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2 comments:

  1. Yes, the connection pattern you’ve decided to go for looks not too hard to achieve, and that electrical engineer guy who did the very interesting article reckoned it was as far as you’d need to go if the batteries weren’t of the especially expensive kind.
    So what’s next – fix alternator, then get & fit batteries?

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