We cycled the old batteries and checked their capacity. The worst gave about 8 hours at 3.6A discharge after one cycle so less than 30Ah.
For the test today we aimed to get a short run at 4kW to get a second data point around the intended cruising power level. As the load increased the voltage was dropping so we were unable to get design rpm from the unit. Top power was 2.8kW. At that we managed 7.5kph but voltage continued to drop.
We set the power back to 2kW to save the batteries and were doing 6.5kph. We ended up 2km out of the marina by the time we had done our testing. We let the batteries recover a bit and did some accelerating and reversing. Then we set the unit at low power (under 1kW) to conserve the batteries and trundled slowly back to the mooring - speed 4.5kph. We managed to get into the marina and pick up the mooring under electric power alone. Overall we covered 4km with these less than ideal batteries.
The linked video shows two conditions - first is at 2kW and then at about 900W:
You can see we had perfect conditions for motoring. There is a photo between the two clips that shows one of the batteries and the basic metering system with voltmeter and DC tong. I did log speed but the GPS is still on the boat (I hope).
We are now waiting on a 48V 60Ah lithium battery bank so we can do higher power tests. Plan is to get one set for testing with the single unit before committing to two permanent units. With a good battery the single unit could enable getting on and off the mooring in winds to about 15kts. To get above that we will need two units and two good batteries.
The boat speed is down on my calm water predictions but the hulls are slimy again and motoring under 5kts is not going to get them clean - and too cold to dive without a wetsuit.
The old batteries were holding up quite well at 19A so the heavy load at full charge today may have improved the capacity but they are still close to useless for this sort of loading. The lithium are far superior for high power application and work out at about half the cost and a fraction of the weight of lead/acid to do the same job.
Rick Willoughby
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