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Krooko
Krooko

Posted: Mon Oct 26, 2015 6:17pm

Post Subject: energy

I have a conventional split 12v / 240 electrical system on my narrowboat with an inverter and 4 leisure batteries. As my 4 x 135 ah batteries are struggling to provide enough power for my 240 v appliances I was thinking of reserving one of the prospective Tesla Powerwalls. This would seem to be a better way to absorb the energy from my roof-mounted 570w solar array. The P'wall could be bolted to the tumblehome below the hatch as I enter the boat, which is unobstructed and close the electrics. Would this be a good idea? thanks for any info-response,

Tony-B
Tony-B

Posted: Mon Oct 26, 2015 7:29pm

Post Subject: energy

1/2 kW of solar and you are running out of power! I note that you give no information about your loads and time they run. There is no way I can comment other than to say it seems to me that you think throwing technology and money at a problem will make it go away. I think it would be far more productive to work through the power/energy audit for YOUR boat and use, then do the battery size calculations and finally the charging calcs. You will find this all explained in the maintenance course notes on www.tb-training.co.uk. Use very conservative figures so assume the loads will be a little higher than any manufacturer says they will be and the time you run them for longer. Assume the charging to be at a lower rate than you think. This is back to basics and I guess you will find that your actual battery capacity is now far lower than what the labels say. It might even prove that 4 batteries are not enough, especially in winter. I find it interesting that you give no details about how you monitor the batteries state of charge so that implies that either you do not or the method is likely to be inaccurate. By all means be an early adopter and maybe count the cost, I seem to remember multimillion dollar Dreamliners catching fire with similar technology. I am not saying the Tesla product will do the same but I would not take the risk. I am saying that whatever you do you need to understand what is going on and then monitor the batteries state of charge on an almost daily basis, switching to shoreline, generator or alternator charging when needed. I suspect a set of 2V traction cells of adequate capacity and properly charged & maintained would last a similar time to the Powerwall and be far easier for a boater to test.

Krooko
Krooko

Posted: Tue Oct 27, 2015 1:04pm

Post Subject: energy

Dear Tony, Many thanks for your response. That is sound advice. Yes, I have a fair amount of power when it's sunny, but most of the energy gets used in the evenings for my lcd tvs, hifi etc. I have a decent E-tracer MPPT regulator, which gives me data for SOC etc. I guess the issue is that I have brought a lot of 240 v appliances in from my old house, and the batteries and 1.6 kw inverter are struggling to power them. As they failed, I'll replace my tv and monitor with 12v leds, and a car stereo instead of the hifi. I have no more space for conventional car batteries, and they don't perform that well in a cold battery locker. They're Enduroline lead-calcium 135s. maybe Trojan Deep cycle batteries would be better. Hence the potential benefit of bringing the Powerwall inside, though it would also fit outside in the hatches of my semi-trad boat. You're right to advise caution - I don't want an electrical fire on the boat! thanks again, MC

Tony-B
Tony-B

Posted: Tue Oct 27, 2015 5:58pm

Post Subject: energy

Thanks, things are becoming a little clearer now. Your batteries use lead-calcium technology, the same as my 3.5 year old Exides in a 330Ah bank running electric fridge, TV, laptop, gas central heating and a mixture of LED & florescent lights I do not have battery problems in the winter. Some will say that I am a bit obsessive about monitoring my batteries state of charge though. I do not believe the temperature makes any significant difference to your problems. If you are using the SOC meter on a solar controller I fear you have been mislead. It is not much more than a glorified voltmeter that makes guesses as to state of charge. When a battery has been rested with no load or charge for several hours the voltage is a good indication of the state of charge of the batteries but the no load and no charge bit is hard to achieve on a boat when living aboard, especially as the solar panels will be elevating the voltage to some degree during daylight. I think that you have left your batteries in a partially discharged state for long periods, probably over last winter. This has caused the chemicals in the batteries to alter so you have lost a lot of capacity (sulphation) so not your 540 Ah batteries will have a much-reduced capacity. The problem is that even if they only have 50 Ah worth of useable chemicals when those chemicals have been charged up the voltage will indicate 100% charged. This catches out many people. I will not rely on the SOC meter on my solar controller and neither should you, the reading is just as likely to be a lie than the truth. When it is dark outside and with virtually no electrical loads turned on you can infer the state of charge from the voltmeter. Fully charged is 12.7 to 12.8 and consider 12.25 as fully discharged. It is not, its about half charged but by doing so you optimise the battery life. The more often and the lower you go below 12.25 the shorter the battery life. However the degree of charge is of the available capacity now, not the capacity when new. If you have a hydrometer (plenty of how to use stuff on the internet but do not get too bogged down in temperature correction) you can compare the state of charge indications from the two instruments. The more they disagree the more capacity you have lost to sulphation. If you have an ammeter that can read the charge flowing into the battery then when charging from a charger or alternator, NOT solar, you can consider the batteries fully charged when the charging current is about 1% of the banks capacity, so in your case around 5 to 6 amps. On a new bank this could take 8 hours or more. Allowing for the state of charge when starting charging the faster the bank charges the greater the loss of capacity. Unfortunately Solar complicates the issue because a falling current can mean the batteries are getting charged up or it might mean the sun has gone in. It is very hard for an ordinary boater to test the capacity of the batteries. All you can do is fully charge one, take it out of circuit and apply a known load. Then time how long it takes for the voltage to drop to about 11.8. Then multiply the load (amps) by the time in hours. A further potential cause is that if you are using a modified sine wave inverter they can cause certain equipment to draw more current and run hot. The UK made Powervault promoted as a cheaper version of the Tesla Powerwall costs between £2000 and £2800. You can buy a lot of batteries for that. Please also note that if you do not keep any lead acid battery fully charged it WILL sulphate be it cheap leisure battery, Trojan, or 2V traction cells. This is why it is so important to really understand your batteries and charging system and then operate the boat to maximise the batteriesâ life.

Krooko
Krooko

Posted: Mon Nov 16, 2015 2:45pm

Post Subject: energy

Tony, many thanks for all the extra detail and advice. My batteries are new and sealed. I take on board the limitations of the SOC reading on my regulator, and may invest in a NASA 2 controller, which can measure current as well as voltage. My batteries are new and sealed. I don't permit them to discharge below 12v, but can switch on the generator when they dip to 12.2 v if you think that it's a good idea. Thanks for the tip on the Powervault. However, I think that it will be too heavy and I have nowhere to put it on the boat. the Powerwall is ideal because it can lie flat against the tumblehome. I'm impressed with lithium technology, having recently invested in a Shorai lithium battery for my motorbike. As soon as I have installed the NASA controller, I'll be able to monitor the charging and current usage. I suspect that my Sterling 1.6 kw pure sine inverter is drawing quite a lot of current as it will drain down the batteries from 12.9 volts to 12.0 in four hours, even with no 240 v appliances being switched on. There may be a drain through the hull too if a wire is shorting out against it - though the impedence registers as 0 when I put a multimeter between the hull and the block that all the black wires are fixed to. I doubt that the batteries have been sulphated in such a short time as I bought them in late August 2015 and they have never been overcharged (charge limit 14.7, bulk 14.v, float 13.8v). However, the charging can be quite high voltage as, even when the floating voltage has dropped to 13.2 v, it gets put back up to 14.4v through my Waeco 1235 charger, which is always fixed up to my generator. The voltage also gets put up to 14.4 when I switch the engine on for hot water. I thought that sealed lead-calcium batteries could not be damaged by sub-14.7 v charging voltages? anyway, everything works - it's just frustrating that I can't sore more of the energy from my 570w of solar. Things may get easier as I gradually switch from 240v to 12v appliances. Thanks again for the tips, regards, MC

Tony-B
Tony-B

Posted: Mon Nov 16, 2015 4:25pm

Post Subject: energy

Some points for you to consider. Unless it is set up correctly AND your batteries are very regularly brought up close to 100% charged the NASA 2 will be just as prone to telling lies as for state of charge as your solar controller is. In fact it will be worse because the error will be accumulative. Put far more faith the getting keeping the charge going until it is around 1% of the battery capacity in amps (So 440Ah of battery look for about 4 amps of charge) from non-solar sources. If the batteries are well charged the Waeco charger should drop to float very soon after it starts charging, if it does not maybe its faulty or the batteries are more discharged than you think. The 14.4 volts from the alternator is fine. I do not understand how a battery bank of 540 Ah can lose over half its charge (12.8 to 12 volts) in four hours with just the inverter standby load. This implies that the batteries have already lost a lot of capacity. Any lead acid battery will lose capacity when left partially discharged and the more discharged and for longer the greater the degree of sulphation. I do not think damage has been done by excessive charging voltage, I think it was sulphation last winter. I have already pointed this out so will say no more about it. Zero ohms resistance between the negative bus bar and the hull is correct. The way to test for leaks is with an ammeter in series with the battery pos. or neg. cable. Turn everything off at the master switches, disconnect a battery lead (any will do) and reconnect it via an ammeter, then turn the power back on. Expect a few 10s of mA. If your generator auto-starts that will also be drawing a little current while it is monitoring battery voltage, as ill any car radio and the inverter unless actually turned off.

Krooko
Krooko

Posted: Sat Nov 21, 2015 7:49pm

Post Subject: energy

Great stuff Tony, particularly testing for leaks. In future, I'll make sure that the batteries are fully charged before i retire for the day, or I leave the boat for any length of time. Will also not let them fall below 12.2v if I can help it. The four 135 ah batteries were purchased this August, so they will not be degraded just yet I hope. All the more reason to look after them. I see that Mastervolt now manufacture lithium marine batteries, but they'll have to fall considerably in cost before I would consider them. Thanks again for your advice, will digest.

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