Tuesday, 31 March 2015

Wind, hail and more on batteries

I managed to get some sleep last night despite the banging on the submerged ledge. Had we gone just a couple of hundred yards further yesterday we would have been able to tie up to some piling with some other boats. Oh well. We had to stop when we did as it was getting just a bit too dark.

This morning we got under way at about 0830, initially in sunshine, but the rain was never far away. This rainbow appeared shortly afterwards, and then came the rain and hail. The wind was pretty strong, but caused no real problems.

The sun did come out now and then, but I had to keep all my layers on at the helm.

The 180 degree turn at Sutton Stop onto the Coventry Canal started well, with the bow being blown the right way, but then the whole boat was being driven diagonally towards the narrows. A blast of astern sorted things out, and we continued to Bedworth where we were given lunch by Christine and Terry. That was very nice, thanks. Terry and I talked about batteries and power. He showed me his power meter, into which I plugged in our fridge to see what it consumed: 80W. This corresponds to approximately 8A from the battery bank, once inverter losses are taken into account. Given that the solar panels were charging at the rate of 9A when the sun shone, and much less when it didn't, it's clear that the fridge relies on the batteries being properly charged by the alternator when the engine is running. I checked the voltage with the engine running: 13.8V. This seemed a bit low - would the batteries ever get fully charged?*

I should have got Terry and Christine to wave when we departed. Perhaps next time.

We had planned to get to Atherstone Top Lock tonight, but I was feeling very tired. When the visitor moorings at Nuneaton came into view I pulled in and tied up just as more hail and heavy rain started to fall. After I snoozed for a while we walked into the town to check out the Maplin power meter Terry had used: £19.99. I didn't buy it. We did spend that much in Sainsbury's, though; then we called in on more friends, Les and Ursula, who are waiting for a crane to lift their boat, Clemency, back into the water at Boot Wharf (Starline). We arranged to return for drinks after tea.

This is when I decided to rewire the battery bank in the method suggested on the SmartGauge website
(thanks for the link, Martin). This ensures that each battery of the four in the bank is used and charged equally, cleverly balancing out problems of cable and connection resistance. I thought I'd have it done by the time we were due at Clemency, but I ran into a little difficulty when I discovered that one of the leads wouldn't reach the terminal it had to connect to. I could solve this by rotating each battery through 180 degrees, effectively making the +ve and -ve terminals swap sides. Too late to connect up - I joined Les and Ursula for a glass or two of wine. I finished off the job when we got back to Jubilee - all seems to be working again.

*Later I discovered that I'd missed off one lead when connecting up the batteries; the temperature sensor lead from the alternator controller. When I reconnected it and ran the engine the voltage went up to 14.1V, which is more like it.


Anonymous said...

Hi Halfie
I've been reading the saga of the new batteries and was intrigued by the debate over how many batteries to fit. Now that you have wired them so that the are used and charged equally wouldn't it make sense to fit the maximum number that you have room to accommodate? That would give you the most capacity and charging would take no longer. Or have I missed something?

Chris (Black Swan)

Halfie said...

Chris, I don't know. The more batteries you have, the less discharged each will be after a night running the fridge. The same overall amount of charge should be used whether you have one battery or ten; and the same amount of charge will have to be restored the next day (from alternator/solar panels etc). With one battery, of course, the danger is that it could become severely discharged which would shorten its life. With the bank of ten, each battery would lose perhaps only a few per cent of charge. But it might be difficult to be sure that each is fully recharged. Ten batteries, of course, cost ten times as much as one (ish - there might be discount for quantity!) So it would appear to make sense to match the ampere hours of the battery bank to the power usage, with a bit in hand. And, probably, factor in replacement every few years. Hope that helps!

Martin said...

About the number of batteries – that was exactly Chris Gibbons' point about how many to fit. Cost effectiveness (he says) is best achieved when you have the number that means they typically get discharged 50% before re-charging. This is not only linked to the cost of the batteries, but to the fact that re-charging to 80% is quick, but from 80 to 100% gets slower and slower.

And about the fridge – did you allow in your calculations for the time when the compressor is off? Although it depends on various factors, I read somewhere that it would be actually running for only 30-50% of the time.

Halfie said...

Martin, 80W is what the fridge was consuming when the compressor was running. I don't know what the average duty cycle is; if it's 50% then that would equate to approximately 4 amps x 24 hours = 96Ah, say 100Ah. We are currently switching off the fridge at night so this cuts it down a bit. A good portion of the actual running time of the fridge is when the engine is also running (and the solar panels are doing their thing), so I doubt whether it really is a huge drain on the batteries.

Tom and Jan said...

The following website has information on the charging of SLA batteries

I think the relevant columns are
Battery Temp Charge Voltage
-20 °C * 16.02 to 16.56
-10 °C * 15.66 to 16.2
0 ° C * 15.3 to 15.9
10 °C 14.94 to 15.54
20 °C 14.58 to 15.18
25 °C 14.40 to 15.00
30 °C 14.22 to 14.82
40 °C 13.86 to 14.46
50 °C 13.5 to 14.10



Halfie said...

Thanks Tom. The website seems to suggest that the batteries could take a higher charge voltage if used in a deep cycle mode, which boat leisure batteries tend to be. The lower figures, maxing out at about 13.8V, seem to be appropriate if the batteries are kept in a "standby mode".