Electric boat conversion - the batteries

Key to the success of our conversion are the special Odyssey batteries. Bear with me, it gets complicated.... (the photo shows 3 of the six we use - the untidy brown wire is to do with metering available battery capacity... complex and unusual... see separate posting.)

All conversions I'd heard of tended to assume one charged ones batteries for a day's cruising and then recharged them overnight - except hybrid systems where, when running along driven "conventionally" by the diesel, it re-charged drive batteries for use when you decided to run "electric."

But the hybrid systems didn't suit my idea - highly complex with clutches and extra alternators... and you needed a perfectly good conventional diesel... we couldn't use our old engine, it was clearly on the edge of falling apart!!!!

No, I wanted a really simple system with the propellor permanently connected to the (electric) drive motor, no belts or clutches.... but how big a battery store would one need?

From other conversion attempts I'd researched and advice and my own (very approximate) theoretical calculations, about 800 A-hr to be sure not to run out of power... although in our case we could probably "get away with" as little as 400 A-hr....

But... where on earth would they go, safely? Without virtually rebuilding the boat, how could one ballast sensibly? ... All advice seemed to be that standard traction lead acid cells worked best... 2V 100 A-hr units, so for 72V and 800 A-hr, this meant 8 x 36 cells.. 288 of them!!! All needing to be slightly spaced for safety reasons. One advisor suggested putting them under the bed.... and then added that they would need a containing lid to careful vent any (hydrogen) gassing out of the cabin to avoid explosion....

I found it difficult to hear this keeping a straight face - the idea of having a potentially exploding bed did not appeal, sheer numbers of cells and their mounting sounded horrific, cable lengths carrying currents to the motor would be quite long for currents approaching 200A....

And then, even worse, they couldn't be recharged (I was told) at more than 20A...

So if we had 8 "columns" (to make up the 800 A-hr) it would need 160A running for 4 hours to recharge the pack from 80% discharge.....

4 hours sitting recharging didn't sound much fun on a quiet countryside mooring. And, besides, the generators I'd researched could only do, say, 80A, and that, pushing it. So, 8 hours recharging, absolutely not on!!

So I puzzled and puzzled trying to think of a solution......

And then I suddenly realised... well, my initial figures suggested we might use about 20A for normal running along ... which I doubled in case of losses I hadn't thought of. (Not a bad estimate... it turns we use 40A for normal running along!) I suddenly realised that if we could recharge at (roughly) 80A including whilst running along then it was a whole new ball game... if I could find batteries that could recharge at the full generator maximum rate then it would become a question of "how much of the time must the generator be running", rather than "how far can one run on a charge"... and, indeed, the battery pack capacity merely had to be enough to make decent chunks of time free to run "pure electric" compared to times running "diesel electric", with the generator powering the motor and recharging the batteries.

I was blissfully unaware that the generator manufacturors and boatyard engineer Alan would nearly have a heart attack at the idea of running the generator whilst taking (potentially highly variable, including more than the generator could provide) current to drive the boat and only found out they thought this might be a problem after I'd perfectly successfully done this...

But that's to look ahead. My theoretical cogitations had got as far as "if I can find a battery-type can recharge at up to, say, 80A, we only need 100A-hr (say)"... where I'd worked out we should probably then have to run the generator about a third of the time... an extraordinarily accurate prediction, it turned out - for canal useage, at least... although it also turned out one needs a bit of extra recharging for true full recharge.

Here I hit an impasse. I knew the idea of such batteries couldn't be silly - hybrid cars exist. But I could never get near any idea of a simple supply and the type used (as far as I could divine from internet search) required terribly complex re-charging circuits.....

I slightly despaired. But then some family friends from "up north" phoned for my birthday (January 2005) to sing "Happy Birthday" down the phone... well, they call it singing. Very sweet of them (and also funny!) Issy asked, as one does "How are you doing?!... I said "Fine except I can't find anybody makes the kind of battery I want...." Issy relayed this to her husband Ali (Alan.) I'm not sure how many beers Ali had had, but I heard him say "Edwin's wittering on about batteries?... I'll find him a supplier..."

In fact all he did was to open their local Yellow Pages and pick out a name (and the phone number) basically at random... I was fully aware of this... but, with no other avenue to pursue, I tried phoning to thereby contact Brian Shaw of DC technologies, in Halifax....

I sketched my conversion plans briefly, verbally... but then suggested I email the whole plan to see if he came up with a battery solution... he kindly studied that, came back with an email saying best bet was... standard traction cells... so I emailed back and said the quivalent of "No, no, I need something quite different... but thanks for the interest..."

Next day.. I'll never forget, another email from Brian. Said he'd "Put his thinking cap on"... what I needed was Odyssey batteries... calmly said they could withstand recharging currents in excess of 200A for a 100 A-hr unit...... and in the flurry of emails that followed, it turned out he could not only supply the things, he'd worked on their development in the USA some 20 years before... so he could tell me ALL about them... what was to become crucial advice about optimising recharge rate and all the rest.

Nevertheless, the key facts came across very fast... and complete solution for us. No gassing when recharging. With that rate of recharge OK, we only needed 100 A-hr - so, 6 x 12V units... and they could even go in the engine space... so no alterations of domestic things, no batteries under the bed, short high-current leads... I raced to do an approximate ballasting calculation...

Oh my goodness, if we removed the existing engine space ballasting, ballasting should be almost exactly as it had been.....

Yet another very approximate calculation that turned out amazingly accurate. And, yes, simply very lucky. My pre-calculations merely showed that there shouldn't be at least a major problem, didn't avoid one that the same calculations might have revealed!

Some technical data: the 100 A-hr units are model PC2150:
Size (lbh) 330 x 173 x 239 (mm) or 13" x 7" x 9.5"
Weight: 34kg per unit (so six weigh 204kg (450 lb.)
Our cost, very approximately £200 per unit... but totally dependent upon the lead market which is going through the roof at the moment.....
Can discharge to 80% with no problem - but can be re-charged from "dead" merely taking longer. Cycle life is around 550 for 80% discharge cycles, but more for lower % cycles. No gassing when recharging for re-charge currents up to >200A, so long as recharge voltage is <15v In fact the way the batteries behave - although completely OK and better than that - is far more complex than this simple data appears to state., but I think that should go in a separate posting about "performance" - I've now had masses of time observing what happens in practice and hope my observations may be of use to others!
Brian's email is brian@dctechnologies.co.uk

I've not met anybody not startled our system works terribly well with such a small battery pack - the idea of cyclical recharging seems to be "foreign" to people considering electric conversions even though WW2 subs (let alone other things) used or use it!.....

Perhaps people want the "ideal" of an electric boat that recharges by magic. That's almost true if you could plug in overnight... but you can't, way out in the countryside. And even not in the countryside, you'd have to be sure to arrange a plug-in point..... maybe some people like the idea of planning their boating holiday in that detail... we feel the a major point of narrowboating is to travel at whim... change one's mind if the weather changes, whatever...

The "cost" of that freedom is that you need to run the generator from time to time....

Not exactly an arduous cost with the generator so silenced (see separate posting) most people scarcely notice whether it is on or not it in the first place! And perhaps I should comment that in people's plans for conversions like ours they seem to forget how much of time "boating" is not actually travelling along......

You wait for locks. You wait in locks. You stop and have lunch. You stop for a walk. You stop at the end of the day and have to find a shop...

Which actually means, we've found, you can boat "electric" almost all the time so long as you turn on the generator "when it doesn't matter".. meaning, you are so busy doing other things or not there at all you don't even have to live with its slight hum and vibration!

Ah, as a boating friend typically teased "So you leave the boat running annoying people moored next to you..."

Um. Well, most people we've asked don't notice whether the generator is on or off at 2 metres away. I always know... but, then, I know whether it is on or off! At 4 metres, I would be being terribly particular to say I can hear it is on....

But the answer to the tease was fact. "Well, if they did notice it was on, and most people don't, it only has to run 40 minutes normally on a mooring at lunch or end of day to be right back to full recharge."

Now that's the beauty of these rapid-recharge batteries. In practice you never go near 80% discharge, just use about 20% and then there's always moments to recharge that... not quite fully, but pretty close. 40 minutes at lunch or end of the day is enough to make up slight losses and do the complicated final stages to be fully "bulk" recharged. 40 minutes when a neighbour would be hard-put to even notice the generator was running...

Metering the batteries to have a decent "at a glance" indication of useable charge left was an electronic nightmare... I'm afraid you can't buy it. I solved the problem... but that's another posting.