Designing a battery system

[Alan de Enfield]

£430 is still cheap. Link please.

 

I cannot find it now, but, this one is close at £449 (inc VAT)

 

http://www.es-store.co.uk/product_details.php?product=CENT%2012/060&description=Victron-Centaur-12-volt-60-amp-battery-chargertop=Battery-Chargers&top_id=21&show_menu=21

[MtB]

One here for £450...

 

http://www.ebay.co.uk/itm/Victron-Centaur-12V-60A-Battery-Charger-/252359679104?hash=item3ac1cf1c80:g:RnUAAOSwqu9VOKX9

[WotEver]

£430 is still cheap. Link please.

£400 here: http://www.ebay.co.uk/itm/like/151920253647?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=122&googleloc=1006966&poi=&campaignid=207297426&device=m&adgroupid=13585920426&rlsatarget=aud-133395220866%3Apla-131843268306&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=122&ff19=0&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F710-134428-41853-0%252F2%253Fipn%253Dicep%2526icep_id%253D67%2526mtid%253D1673%2526kwid%253D1%2526crlp%253D50600204586_563391%2526icep_item_id%253D151920253647%2526itemid%253D151920253647%2526icep_meta_categ_id%253D131090%2526icep_etrs%253DY%2526icep_epid%253D-999%2526icep_ctlg%253D-999%2526icep_cond%253DNew%252Bother%252B%252528see%252Bdetails%252529%2526targetid%253D131843268306%2526rpc%253D0.15%2526rpc_upld_id%253D69305%2526device%253Dm%2526icep_msku_flag%253Dn%2526icep_cbt%253Dn%2526adtype%253Dpla%2526googleloc%253D1006966%2526poi%253D%2526campaignid%253D207297426%2526adgroupid%253D13585920426%2526rlsatarget%253Daud-133395220866%253Apla-131843268306%2526gclid%253DCjwKEAjwgPe4BRCB66GG8PO69QkSJAC4EhHhszMwZE5m8_T70EPIay9yQeogcKndjCMYVvpUe5bJ8hoCcc3w_wcB%2526srcrot%253D710-134428-41853-0%2526rvr_id%253D1020155854729&limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=122&chn=ps&googleloc=1006966&poi=&campaignid=207297426&device=m&adgroupid=13585920426&rlsatarget=aud-133395220866%3Apla-131843268306&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=122&ff19=0&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F710-134428-41853-0%252F2%253Fipn%253Dicep%2526icep_id%253D67%2526mtid%253D1673%2526kwid%253D1%2526crlp%253D50600204586_563391%2526icep_item_id%253D151920253647%2526itemid%253D151920253647%2526icep_meta_categ_id%253D131090%2526icep_etrs%253DY%2526icep_epid%253D-999%2526icep_ctlg%253D-999%2526icep_cond%253DNew%252Bother%252B%252528see%252Bdetails%252529%2526targetid%253D131843268306%2526rpc%253D0.15%2526rpc_upld_id%253D69305%2526device%253Dm%2526icep_msku_flag%253Dn%2526icep_cbt%253Dn%2526adtype%253Dpla%2526googleloc%253D1006966%2526poi%253D%2526campaignid%253D207297426%2526adgroupid%253D13585920426%2526rlsatarget%253Daud-133395220866%253Apla-131843268306%2526gclid%253DCjwKEAjwgPe4BRCB66GG8PO69QkSJAC4EhHhszMwZE5m8_T70EPIay9yQeogcKndjCMYVvpUe5bJ8hoCcc3w_wcB%2526srcrot%253D710-134428-41853-0%2526rvr_id%253D1020155854729&ul_noapp=true

 

Loads more around for about £435 - just Google Victron Centaur 12/60.

[Dave_P]

£400 here: http://www.ebay.co.uk/itm/like/151920253647?limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=122&googleloc=1006966&poi=&campaignid=207297426&device=m&adgroupid=13585920426&rlsatarget=aud-133395220866%3Apla-131843268306&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=122&ff19=0&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F710-134428-41853-0%252F2%253Fipn%253Dicep%2526icep_id%253D67%2526mtid%253D1673%2526kwid%253D1%2526crlp%253D50600204586_563391%2526icep_item_id%253D151920253647%2526itemid%253D151920253647%2526icep_meta_categ_id%253D131090%2526icep_etrs%253DY%2526icep_epid%253D-999%2526icep_ctlg%253D-999%2526icep_cond%253DNew%252Bother%252B%252528see%252Bdetails%252529%2526targetid%253D131843268306%2526rpc%253D0.15%2526rpc_upld_id%253D69305%2526device%253Dm%2526icep_msku_flag%253Dn%2526icep_cbt%253Dn%2526adtype%253Dpla%2526googleloc%253D1006966%2526poi%253D%2526campaignid%253D207297426%2526adgroupid%253D13585920426%2526rlsatarget%253Daud-133395220866%253Apla-131843268306%2526gclid%253DCjwKEAjwgPe4BRCB66GG8PO69QkSJAC4EhHhszMwZE5m8_T70EPIay9yQeogcKndjCMYVvpUe5bJ8hoCcc3w_wcB%2526srcrot%253D710-134428-41853-0%2526rvr_id%253D1020155854729&limghlpsr=true&hlpv=2&ops=true&viphx=1&hlpht=true&lpid=122&chn=ps&googleloc=1006966&poi=&campaignid=207297426&device=m&adgroupid=13585920426&rlsatarget=aud-133395220866%3Apla-131843268306&adtype=pla&crdt=0&ff3=1&ff11=ICEP3.0.0-L&ff12=67&ff13=80&ff14=122&ff19=0&ul_ref=http%253A%252F%252Frover.ebay.com%252Frover%252F1%252F710-134428-41853-0%252F2%253Fipn%253Dicep%2526icep_id%253D67%2526mtid%253D1673%2526kwid%253D1%2526crlp%253D50600204586_563391%2526icep_item_id%253D151920253647%2526itemid%253D151920253647%2526icep_meta_categ_id%253D131090%2526icep_etrs%253DY%2526icep_epid%253D-999%2526icep_ctlg%253D-999%2526icep_cond%253DNew%252Bother%252B%252528see%252Bdetails%252529%2526targetid%253D131843268306%2526rpc%253D0.15%2526rpc_upld_id%253D69305%2526device%253Dm%2526icep_msku_flag%253Dn%2526icep_cbt%253Dn%2526adtype%253Dpla%2526googleloc%253D1006966%2526poi%253D%2526campaignid%253D207297426%2526adgroupid%253D13585920426%2526rlsatarget%253Daud-133395220866%253Apla-131843268306%2526gclid%253DCjwKEAjwgPe4BRCB66GG8PO69QkSJAC4EhHhszMwZE5m8_T70EPIay9yQeogcKndjCMYVvpUe5bJ8hoCcc3w_wcB%2526srcrot%253D710-134428-41853-0%2526rvr_id%253D1020155854729&ul_noapp=true

 

Loads more around for about £435 - just Google Victron Centaur 12/60.

I'd rather pay a little more to buy from a reputable supplier. I've heard too many eBay horror stories. eBay is good for very cheap stuff and used auctions. I wouldn't hand over £400 in this case. If something goes wrong I want to be able to go up to a counter and hand it back. Maybe I'm old fashioned.

[WotEver]

I'd rather pay a little more to buy from a reputable supplier. I've heard too many eBay horror stories. eBay is good for very cheap stuff and used auctions. I wouldn't hand over £400 in this case. If something goes wrong I want to be able to go up to a counter and hand it back. Maybe I'm old fashioned.

Your choice, but from Ebay there is a full address and the seller has a good feedback rating, COASTALRIDES IS A UK VAT REGISTERED COMPANY, ALSO A UK MAIN AUTHORISED DEALER IN DOVER MARINA KENT FOR, YANMAR, VETUS AND MERCRUISER.

[tonyt40]

Another option :

 

Midland Chandlers : http://www.ebay.co.uk/itm/Waeco-Perfectcharge-12-Volt-50-Amp-Battery-Charger-MCA1250-Narrowboat-Caravan-/272203750976?hash=item3f609b9e40:g:FN4AAOSw8d9U0Ks0

 

 

You are correct - having looked again (very closely) that is excluding VAT, so it comes out at around £430.

 

Tricky blighters some of the chandleries.

 

However, the Waeco one at £265 includes VAT and delivery - that is good value.

 

The difference in charging times between a 50 amp and a 60 amp is probably only a matter of a few minutes, as very little time is actually spent charging at full rate, and then only on 'flat batteries'. As soon as the batteries get 'a bit' of charge in them the current drops and you will find that you are actually putting in (maybe) 10 amps for many hours, rather than 50 or 60 amps for an hour. (its the way batteries 'work')

Thanks for that Alan. £400 with a box or £265 without - now let me think? I will get this and fit it then at least I can charge from the generator. Seems like a good buy without breaking the bank! Cheapest I can find anywhere else is about £350.

[tonyt40]

So what about battery monitoring then? Is the Smartguage worth it or do I look at other options?

[WotEver]

So what about battery monitoring then? Is the Smartguage worth it or do I look at other options?

I'll go get me popcorn.

 

If you have an accurate voltmeter and ammeter and (important bit) know how to interpret their readings (or are prepared to take the time to learn) then no, you don't need a SmartGauge. If you just want a simple 'fuel gauge' and not have to worry about such things then a SmartGauge might suit you better.

 

Whether or not you get a SmartGauge you'll need an ammeter.

 

Tony

[MtB]

 

Whether or not you get a SmartGauge you'll need an ammeter.

 

 

 

Why?

I have a Smartgauge and find no need for an ammeter.

[tonyt40]

Am I right in thinking that the DC system requires a battery isolator on the output to the inverter and 12v distribution (as well as to the engine - where they are already fitted)? If my understanding is right this is part of the BSS.

 

Thanks Tony

[Tony Brooks]

No, at least no, not as written.

 

The output of the inverter is AC at mains voltage so it has nothing to do with 12V BUT it may be that you fit a dedicated inverter 12V supply from the battery and that will need an isolator. If your existing 12V isolator and battery wiring is man enough (probably not though) you could feed the inverter from its switched side.

 

Best proactive dictates that you need an RCD and circuit breakers on the inverter output side, this may well have a mains type master switch. If you have a shore line you really should have an isolation transformer or galvanic isolator unless you want to risk your hull developing holes.

 

The battery master switch may be a single one in a negative lead (not a good idea) or a pair, one for each bank, fitted "as close as practical" to the batteries in the two banks positives.

 

The type of 12V fuse/circuit breaker distribution box you choose may or may not have its own isolator, s fara s I know it is not mandatory but the master switches are.

[Arthur Brown]

Most of your electricity needs will be shown if you do an energy or power audit. It's a theoretical exercise in which you decide what electricity you will consume each day by adding up all the amp hours everything consumes while you will have it switched on. After that you look at where that electricity will come from.

 

If you are summer cruising then solar and engine generated electricity will cover your needs, If you are winter live aboards then expect to burn diesel to make electricity during the day.

[WotEver]

Why?

 

I have a Smartgauge and find no need for an ammeter.

Then you'll never know for sure when your batteries have reached close to 100% (unless you measure specific gravity).

 

Tony

[cuthound]

Why?I have a Smartgauge and find no need for an ammeter.

Then you cannot measure the tail current to establish when the battery is fully charged.

 

The only other reliable method is taking sg's, but that is not possible on sealed batteries.

[MtB]

Aha, so am I to understand I can't rely on my Smartgauge then?

[nicknorman]

Aha, so am I to understand I can't rely on my Smartgauge then?

I'm sure you are only being norty but anyway, if you read the Smartgauge spec, it is only accurate to 10% during charge (much more accurate during discharge). Whether this is an issue depends on your charging pattern. If like us you tend to cruise for much longer than necessary to get to 100%, it is of no consequence because at the end of the 7 hour day (or whatever) the batteries will be fully charged by most measures. However if your primary existence is static and you are charging without moving, the aim is surely to know the optimum time to stop that noisy genny etc and for that, the SG is not particularly good. It is OK but a more optimal solution is to check the tail current.

 

So in summary the SG is very good during discharge, not so good during charge because it is "seeing" the charging system voltage not the intrinsic battery voltage and can therefore only make a guess at the SoC based on charging voltage profile and time.

 

As well as it having to guess the SoC, it is also trying to guess something that is not defined - ie 100% Soc

 

Some folk think 100% SoC is when the tail current is 4% (Victron et al) some think it 2%, some 1%, some 0.5% and some like me think that 100% SoC is only asymptotically approached and so can never feasibly be reached.

 

By knowing the tail current you can decide which of the many definitions of 100% SoC you want to go with. In my experience the SG tends to get to 100% at 3 or 4% tail current which is quite good under the circumstances, but I prefer to get a much lower tail current before I would consider the batteries "adequately charged".

Edited April 26, 2016 by nicknorman

[cuthound]

 

Some folk think 100% SoC is when the tail current is 4% (Victron et al) some think it 2%, some 1%, some 0.5% and some like me think that 100% SoC is only asymptotically approached and so can never feasibly be reached.

 

Completely agree, there will always be some tail current simply because every battery has a self discharge current so the battery can never be completely fully charged.

[MtB]

I'm sure you are only being norty but anyway, if you read the Smartgauge spec, it is only accurate to 10% during charge (much more accurate during discharge). Whether this is an issue depends on your charging pattern. If like us you tend to cruise for much longer than necessary to get to 100%, it is of no consequence because at the end of the 7 hour day (or whatever) the batteries will be fully charged by most measures. However if your primary existence is static and you are charging without moving, the aim is surely to know the optimum time to stop that noisy genny etc and for that, the SG is not particularly good. It is OK but a more optimal solution is to check the tail current.

 

So in summary the SG is very good during discharge, not so good during charge because it is "seeing" the charging system voltage not the intrinsic battery voltage and can therefore only make a guess at the SoC based on charging voltage profile and time.

 

As well as it having to guess the SoC, it is also trying to guess something that is not defined - ie 100% Soc

 

Some folk think 100% SoC is when the tail current is 4% (Victron et al) some think it 2%, some 1%, some 0.5% and some like me think that 100% SoC is only asymptotically approached and so can never feasibly be reached.

 

By knowing the tail current you can decide which of the many definitions of 100% SoC you want to go with. In my experience the SG tends to get to 100% at 3 or 4% tail current which is quite good under the circumstances, but I prefer to get a much lower tail current before I would consider the batteries "adequately charged".

 

 

Busted!

 

But regarding gennys, tail currents, ammeters etc, it all seems irrelevant now as I haven't run my genny for many weeks - the solar does the job admirably instead, according to my Smartgauge at least! It just says 100% every night when I get home. Are you saying it is kidding me and just guessing? How clever is it really, when monitoring a solar-charged system, given the system voltage is randomly and continuously varying with sun intensity/absence?

 

I'm sure I'll change my tune again come November though!

Edited April 26, 2016 by Mike the Boilerman

[WotEver]

 

My Smartgauge just says 100% every night when I get home. How clever is it really, when monitoring a solar-charged system, given the system voltage is randomly and continuously varying with sun intensity/absence?

 

None of that latter stuff is really relevant. Your batts will be at 100% this time of year, and SmartGauge is correctly reporting that fact.

 

Tony

[tonyt40]

No, at least no, not as written.

 

The output of the inverter is AC at mains voltage so it has nothing to do with 12V BUT it may be that you fit a dedicated inverter 12V supply from the battery and that will need an isolator. If your existing 12V isolator and battery wiring is man enough (probably not though) you could feed the inverter from its switched side.

 

Best proactive dictates that you need an RCD and circuit breakers on the inverter output side, this may well have a mains type master switch. If you have a shore line you really should have an isolation transformer or galvanic isolator unless you want to risk your hull developing holes.

 

The battery master switch may be a single one in a negative lead (not a good idea) or a pair, one for each bank, fitted "as close as practical" to the batteries in the two banks positives.

 

The type of 12V fuse/circuit breaker distribution box you choose may or may not have its own isolator, s fara s I know it is not mandatory but the master switches are.

Its probably just the way I explained it. the domestic bank is split so there are 3 batteries on each side under the back deck. they are all connected in parallel with longer leads run across the back between the banks. The alternator is connected through a battery isolator on the starboard side. The inverter and domestic 12v distribution are taken from the port side straight from the terminals with no isolator (although there is a mega fuse on the domestic) but it means the only way to isolate then invertor and domestic is by taking the leads off. the bilge pump is also taken straight off the bank although it has a inline fuse fitted. the current battery isolators are adjacent to the bank but not easy to get at without contortionism!

 

I am thinking it needs a bit of a redesign to get some new battery isolators in a position where I can get to them in an emergency and that isolate all 12 v supplies.

[WotEver]

The domestic distribution board should indeed be fed by an isolator. If the inverter is a charger/inverter then it needs to remain permanently connected to the batteries, if not then it also needs to be on the switched side of the isolator, but ensure that the isolator is a good quality one, rated to take the full inverter load continuously.

 

Tony

Additionally, ideally, the alternator should be on the same connection as everything else.

[tonyt40]

Thanks Tony. You confirmed my thoughts. Do you know where I would find the specification for cable for the DC installation.

[WotEver]

Thanks Tony. You confirmed my thoughts. Do you know where I would find the specification for cable for the DC installation.

Here's something wot I writ for another site. Let me know if it just raises more questions than it answers.

 

What size cable

There are two main considerations when deciding on the size of cable for any service: the current carrying capacity of the cable and the voltage drop along the length of the cable.

 

Maximum Current capacity

 

When wiring 'mains voltage' circuits, the limiting factor is typically the current carrying capacity of the cable before the heating effect causes the cable to overheat. This may also be the case for high-power low voltage DC circuits, such as those feeding starter motors and mains inverters, however the size dictated to avoid excessive voltage drop is likely to be far in excess of the current limits. See below.

 

Voltage Drop

 

Over a distance, voltage is lost due to the resistance of the cable. When wiring 'mains voltage' typically this effect is negligible for all but very long cable runs as the loss is likely only to be a few volts at most, however as the voltage decreases these 'few volts' become significant.

 

Typically for a 12V or 24V system you will only be concerned with voltage drop, as the voltage 'lost' will become significant well before you get close to the cable's current rating.

 

Calculating Voltage Drop

 

The voltage drop is caused by the resistance of the cable; the smaller the cable, the higher the resistance (the resistance is inversely proportional to the cable cross-sectional area). For certain items, such as the horn, a voltage drop of around 1.5V probably won't matter. Lights would be better with a maximum drop of 1V.

 

Sometimes we may find that our 'ideal' cable becomes ridiculously large, such as if you wanted a 1 metre cable from a 170A alternator to only drop 0.01V - the cable would come out at 180mm². In these instances we take a balanced view that 90mm² or maybe even 70mm² would suffice, and accept that the voltage drop will be 0.1V at full power.

 

The size of cable required can be calculated with the following simple formula:

cablesize in mm² = 18*metres*amps/(V*1000)

 

Where metres = entire cable length there and back and V = Maximum permitted voltage drop.

 

To work the other way, and see what voltage drop you can expect for a specified cable size and load, rearrange the equation thus:

V = 18*metres*amps/(cablesize in mm²*1000)

 

Let's find the ideal cable size using the example of a 50W tunnel light (4A) on a 60 ft boat with 12V electrics. We want the maximum voltage drop to be 1V. The length of run of the cable (there and back) is 40 metres. Plugging those figures into the above equation gives us...

 

(18*40*4)/(1*1000) = 2880/1000 = 2.88. Round that up and we find that we require 3mm² cable.

 

Another example, this time a split-charge cable. It's 250mm long, carries a maximum charging current of 100A and we want a voltage drop of no more than 0.01V.

 

(18*0.5*100)/(0.01*1000) = 900/10 = 90mm².

 

Fridge cables

 

A 12V fridge with a Danfoss compressor (fitted to the vast majority of 12V fridges) requires 1mm² for each metre run from battery to fridge. So if the fridge is 5 metres away from the battery it requires 5mm² cable.

 

Edited for formatting

Edited April 26, 2016 by WotEver

[tonyt40]

Here's something wot I writ for another site. Let me know if it just raises more questions than it answers. What size cable

There are two main considerations when deciding on the size of cable for any service: the current carrying capacity of the cable and the voltage drop along the length of the cable. Maximum Current capacity

When wiring 'mains voltage' circuits, the limiting factor is typically the current carrying capacity of the cable before the heating effect causes the cable to overheat. This may also be the case for high-power low voltage DC circuits, such as those feeding starter motors and mains inverters, however the size dictated to avoid excessive voltage drop is likely to be far in excess of the current limits. See below.Voltage Drop

Over a distance, voltage is lost due to the resistance of the cable. When wiring 'mains voltage' typically this effect is negligible for all but very long cable runs as the loss is likely only to be a few volts at most, however as the voltage decreases these 'few volts' become significant.

Typically for a 12V or 24V system you will only be concerned with voltage drop, as the voltage 'lost' will become significant well before you get close to the cable's current rating. Calculating Voltage Drop

The voltage drop is caused by the resistance of the cable; the smaller the cable, the higher the resistance (the resistance is inversely proportional to the cable cross-sectional area). For certain items, such as the horn, a voltage drop of around 1.5V probably won't matter. Lights would be better with a maximum drop of 1V.

Sometimes we may find that our 'ideal' cable becomes ridiculously large, such as if you wanted a 1 metre cable from a 170A alternator to only drop 0.01V - the cable would come out at 180mm². In these instances we take a balanced view that 90mm² or maybe even 70mm² would suffice, and accept that the voltage drop will be 0.1V at full power.

The size of cable required can be calculated with the following simple formula:

cablesize in mm² = 18*metres*amps/(V*1000)

Where metres = entire cable length there and back and V = Maximum permitted voltage drop.

To work the other way, and see what voltage drop you can expect for a specified cable size and load, rearrange the equation thus:

V = 18*metres*amps/(cablesize in mm²*1000)

Let's find the ideal cable size using the example of a 50W tunnel light (4A) on a 60 ft boat with 12V electrics. We want the maximum voltage drop to be 1V. The length of run of the cable (there and back) is 40 metres. Plugging those figures into the above equation gives us...

(18*40*4)/(1*1000) = 2880/1000 = 2.88. Round that up and we find that we require 3mm² cable.

Another example, this time a split-charge cable. It's 250mm long, carries a maximum charging current of 100A and we want a voltage drop of no more than 0.01V.

(18*0.5*100)/(0.01*1000) = 900/10 = 90mm². Fridge cables

A 12V fridge with a Danfoss compressor (fitted to the vast majority of 12V fridges) requires 1mm² for each metre run from battery to fridge. So if the fridge is 5 metres away from the battery it requires 5mm² cable.

Edited for formatting

Thanks for that Tony. That's clarified the sizing. When I have sized the cables what should I be looking for - obviously flexible and stranded but anything specific?

[WotEver]

Hey Tony, Thin wall PVC from somewhere like this should be fine ?

http://www.vehicle-wiring-products.eu/section.php/48/1/cables/5273f6734c9aaed8719dc8c560b0f1e8