You cruise sev­eral hours a day, you’ve got a de­cent al­ter­na­tor, a bat­tery mon­i­tor and an ad­vanced charg­ing man­ager – but still you keep killing bat­ter­ies. That’s when a power au­dit and charg­ing cal­cu­la­tion could help to get to the bot­tom of things…

Canal Boat - - Contents -

How do you know if your bat­tery bank and charg­ing sys­tem are ad­e­quate? Do a power au­dit and charg­ing cal­cu­la­tion

This ar­ti­cle be­gan as a ques­tion from boater Val to our ‘Ask the ex­perts’ col­umn. But fol­low­ing Tony Brooks’ ini­tial re­sponse and re­quest for fur­ther in­for­ma­tion from Val, it de­vel­oped into some­thing that we felt de­served an ar­ti­cle of its own: a ful­ly­worked ‘real life’ ex­am­ple of how car­ry­ing out a power au­dit can high­light short­com­ings in an elec­tri­cal sys­tem which at first sight might seem to pro­vide ad­e­quate power and charg­ing ca­pac­ity…

Val bought her boat back in 2012, know­ing that the do­mes­tic bat­ter­ies needed chang­ing. But then six months af­ter buy­ing new ones (7 x 110Ah), per­for­mance started to de­cline again. An en­gi­neer con­firmed that the al­ter­na­tor and ca­bling were fine, but the bat­ter­ies weren’t hold­ing a charge – so they were changed again.

She then started to change things in the boat: fit­ted a new 240V fridge freezer, fed by a Vic­tron 3000W in­verter, and bought an au­to­matic wash­ing ma­chine. Then the en­gine was changed for new one which had a 175A al­ter­na­tor, and at the same time, Val fit­ted a Ad­verc ad­vanced bat­tery man­age­ment sys­tem and a bat­tery mon­i­tor – plus 3 x 150W so­lar pan­els. The third set of bat­ter­ies lasted for 14 months be­fore they too would not hold a charge.

And all this is de­spite run­ning the en­gine for ap­prox­i­mately five to six hours per day. She ended up on her fifth set of bat­ter­ies, tak­ing refuge in a ma­rina with shore­line sup­ply be­cause once again they were

fail­ing to hold charge – and was con­sid­er­ing in­creas­ing the size of the bat­tery bank to eight, and step­ping up from 110Ah to 135Ah to see if this would ex­tend their life. At this point she con­tacted Canal Boat’s Ask the Ex­perts col­umn…

Tony Brooks’ ini­tial re­sponse was to ask for de­tailed fig­ures to en­able him to do a power au­dit and charg­ing cal­cu­la­tions, but also to make two im­me­di­ate com­ments, adding the ad­vice “On no ac­count buy more bat­ter­ies un­til this is checked: you could eas­ily de­stroy them in weeks.”

Firstly, an ad­vanced bat­tery man­age­ment sys­tem can help hide prob­lems in a charg­ing cir­cuit and will re­duce the time taken to get the bat­ter­ies to maybe 80% of fully charged, but af­ter that it is un­likely to re­duce the time to 100% charged. I doubt your charg­ing cir­cuit had any faults.

Se­condly, in gen­eral your type of bat­tery mon­i­tor has a long and dis­rep­utable his­tory of en­cour­ag­ing the less elec­tri­cally aware boaters to de­stroy their bat­tery banks by over dis­charg­ing them and never fully charg­ing them. Un­less you fully un­der­stand how to set the me­ter up and when and how to resyn­chro­nise it I urge you to only use its volt­age and cur­rent scales. It is also safe to use the Amp hours used scale to cal­cu­late the cur­rent bat­tery ca­pac­ity.

On re­ceiv­ing the fig­ures, Tony then went on to do the power au­dit and charg­ing cal­cu­la­tions. But first an ex­pla­na­tion of how bat­ter­ies de­grade, and what you can do to slow the process…

If you do not very reg­u­larly fully recharge your bat­ter­ies a chem­i­cal change known as sul­pha­tion will take place that grad­u­ally re­duces their ca­pac­ity. All bat­ter­ies sul­phate in time, but that time can be a few weeks if they are never prop­erly charged. Also if you reg­u­larly deeply dis­charge bat­ter­ies, you will quickly use up their cyclic life re­sult­ing in in­ter­nal short cir­cuits.

Both sul­pha­tion and short cir­cuits re­sult in the bat­ter­ies go­ing flat far faster then they did when new, so the ques­tion is how do you min­imise these ef­fects. Deal­ing with sul­phat­ing is easy: fully recharge very reg­u­larly, at not more than weekly in­ter­vals. Loss of cyclic life can be re­duced by never fully dis­charg­ing the bat­ter­ies: try to never drop be­low about 50% of fully charged.

The eas­i­est way for an or­di­nary boater to dif­fer­en­ti­ate be­tween sul­pha­tion or run­ning out of cyclic life faults is to look into each cell when on charge (use eye pro­tec­tion for acid splashes). If any are drier than the rest or if any are bub­bling a lot more than the rest then these cells are likely to be short­ing and they are also likely to be hot­ter. If you mea­sure and record the spe­cific grav­ity/rel­a­tive den­sity of the elec­trolyte in each cell and then com­pare it to the state of charge shown on a volt­meter they should both in­di­cate a sim­i­lar state of charge. If the volt­age in­di­cates a higher charge than the spe­cific grav­ity then the cell is sul­phated. If the ends of a bat­tery are bulging out it also in­di­cates a sul­phated bat­tery.

How do you know how fully charged the bat­ter­ies are? The way to make sure they are fully charged is to charge un­til the charg­ing cur­rent at 14.4 volts or more has dropped to about 1% to 2% of bat­tery ca­pac­ity or has stopped drop­ping for about an hour.

You can use the bat­ter­ies’ rested volt­age to in­fer the state of charge. Rested volt­age means be­ing off charge for at least an hour, or be­ing off charge and a rea­son­able load has been drawn for a short while. You also need to en­sure the so­lar ar­ray is cov­ered or its dark oth­er­wise the so­lar volt­age will dis­tort the fig­ures. Typ­i­cal val­ues are shown here:

Now on to the power au­dit. Tony took the fig­ures sup­plied by Val which gave the amps drawn (from the la­bels on the equip­ment, if nec­es­sary di­vid­ing watts by volts to give amps) and hours used per day for each elec­tri­cal ap­pli­ance on the boat. These were mul­ti­plied these to­gether to give the amp hours for each ap­pli­ance, and then added then up to give the to­tal amp hours used per day.

Tony ad­mit­ted to be­ing scep­ti­cal about some of the fig­ures: “I also sus­pect some of the times are a bit high, but let’s take those

fig­ures as cor­rect for now.” He con­tin­ued…

That to­tal of al­most 282 Ah may sound well within the ca­pac­ity of 7 x 110 Ah bat­ter­ies – but re­mem­ber the com­ments above about avoid­ing sul­pha­tion and short cir­cuits. You should try not to let the bat­ter­ies fall be­low 50% of fully charged to op­ti­mise the cyclic life so the min­i­mum bank size for 282 Amp hours of load per day is 564 Ah. How­ever they will still sul­phate and it is of­ten claimed that to al­low for that it is bet­ter to have a daily load of only 25% of bat­tery ca­pac­ity - so that gives a bank size re­quired as 1128 Ah.

But that’s only the first part of the cal­cu­la­tion. You need to make sure that not only do your bat­ter­ies have enough ca­pac­ity to last the day, but your charg­ing setup is ca­pa­ble of putting them back to fully charged. And the charg­ing cal­cu­la­tion isn’t quite as straightforward as it might seem ei­ther…

You can­not get all the cur­rent you put into a bat­tery con­verted to stored elec­tri­cal en­ergy. You will find the ef­fi­ciency of con­ver­sion (mea­sured in terms of how much ex­tra en­ergy you need over and above the bat­tery ca­pac­ity) is quoted as any­thing be­tween 10% and about 40% de­pend­ing upon tem­per­a­ture, age of the bat­tery, bat­tery de­sign and what the per­son quoting the fig­ures is try­ing to sell you. I usu­ally take 30% but as I sus­pect the daily con­sump­tion fig­ures are a bit high I will com­pen­sate by us­ing 20% in this case. So the ac­tual num­ber of amp hours from the al­ter­na­tor that it would take to re­place your daily us­age is 120% of 282 Ah, which makes it 338 Ah.

Al­ter­na­tors can­not pro­duce their max­i­mum out­put for very long. As the bat­ter­ies start to charge, the cur­rent drops so they can only av­er­age about 50% of their out­put over three to four hours. Any time longer than that sim­ply re­duces the av­er­age and makes it dif­fi­cult to cal­cu­late.

You al­ter­na­tor is rated at 175 amps so over three to four hours (no more) that av­er­ages about 87 amps.

338 Ah taken out and charg­ing at an av­er­age of 87 amps = 3.9 hours. But the charg­ing for five or six hours each day looks as if it will get the bank to around 80% charged but not 100% be­cause to fully charge a bank of any sig­nif­i­cant size is likely to take 12 hours plus (re­mem­ber go by tail cur­rent, not your me­ter).

So Tony’s con­clu­sions from the cal­cu­la­tions were:

Firstly, you have been al­low­ing your bat­ter­ies to sul­phate by un­der charg­ing al­though you have done well com­pared with many with the length of time you charge for.

Se­condly, your bat­tery bank is se­ri­ously un­der­sized for your elec­tri­cal load so it is al­most cer­tain that you have been over dis­charg­ing them and have used up the cyclic life.

If, as you sug­gest, you go up to 8 x 135 Ah = 1080 Ah, that would be more than ad­e­quate for hav­ing the bank twice as large as the load – but still not large enough to meet the com­mon rule of thumb of four times the load. Re­mem­ber that as time goes on the ac­tual ca­pac­ity of the bank will drop so with twice the load you will start to dis­charge them to be­low 50% within a few months.

Do re­mem­ber, though, that is based on the­ory that has a lot of vari­ables in it, so take my work­ings as in­dica­tive to give you an idea of what is prob­a­bly wrong.

1128 Ah of bat­ter­ies is ex­pen­sive and even more so as you need a de­cent cyclic life. If you can get to all but fully recharge ev­ery day or to about 80% (not on the me­ter) and then 100% once a week then I would sug­gest that you cost some deep cy­cle bat­ter­ies, Tro­jan 105s are well spo­ken of but they are 6 volts so need pair­ing in se­ries to give 12V. If you can­not en­sure proper charg­ing then stay with rel­a­tively cheap wet open cell bat­ter­ies, and ac­cept that with your loads and charg­ing you will need to change them fre­quently.

I also think that you need to take a very hard look at re­duc­ing the load:

• Why any­one is us­ing halo­gen bulbs to­day is be­yond me, es­pe­cially when they have bat­tery prob­lems.

• I have no idea why a boat needs a kitchen pump but not a shower pump. The sink nor­mally ex­its through the hull by grav­ity.

• Un­less you have a very cheap fridge your fridge con­sump­tion looks very high to me. I would ex­pect it to be 10 to 20 Ah lower.

• I can not see how a We­basto mo­tor and fuel pump can use more elec­tric­ity than a wa­ter pump. From some data I found on­line: - Pump mo­tor 14 watts, fan mo­tor & fuel pump at full load 28 watts = 42 watts so 42 watts at 12 V = 3.5 Amps, that is 27 Ah and part of the time it will be on part load so re­duc­ing that fig­ure. In truth it is prob­a­bly only draw­ing about half of what your power au­dit sug­gests. • I also doubt the toi­let runs for 3 hours a day. Fi­nally, to try to pull this all to­gether: You need a new bank so put it on, but mon­i­tor it very closely. 5 X 135 qual­ity bat­ter­ies will do for now.

Recharge each day un­til the charg­ing cur­rent has fallen to about 1% to 2% of the bank ca­pac­ity (so for 5 x 135Ah that would be 6 to 12 amps) at 14.4 volts or more. Zero your amp hour counter. Read the Ah counter be­fore start­ing the next day: that will give you your ac­tual con­sump­tion with­out the en­gine pro­vid­ing any elec­tric­ity.

From that you can work out the bank ca­pac­ity you need for op­ti­mum life and act ac­cord­ingly but I em­pha­sise that you need to fully recharge as of­ten as you can and at no more than weekly in­ter­vals.

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