Novice paranoia: USB device drain on 12v batteries?

[Darwining]

Hello! I bought a boat with a friend and we have very little idea what we're doing! A Hurley 24/70, with two 12v leisure batteries for electrical, powered solely by the diesel inboard engine (High hopes to kit her out for solar! Eventually...). Aside from instruments and lighting, a single 12v plug wired right into the batteries: no switch for the plug aside from the selector switch.

As we are in the process of moving to live aboard her full time, I thought it prudent to snag a cheap 2 usb car charger for the 12v. A dinky little thing with an output of 5V/2.1A. We aren't planning on running anything with a major draw on it, just phones, charging bank, or maybe a chromebook with a micro-usb cable. Any tips for beginning battery maintenance to avoid running our batteries dry?

I don't have much of an electrical background, but I have been trying to read up on the theory at least, learning the importance of knowing the Watt, amps, volts, etc. etc. (although at times knowing why they matter/what exactly they mean gets REAL hard for my poor brain). I'm sure I'm not the first or the last to have this question, but I thought I'd ask anyway.

Cheers!

 

[MarkCX]

This would probably be better asked in the “reader to reader” forum, some really knowledgeable people there who don’t necessarily come into the “live aboard” forum, and are much better qualified than me!

As for basics, in simple terms, running your batteries down to below 50% will almost certainly kill them really quickly. A very rough and ready way to check is by measuring the battery voltage when they’re not powering anything. 12.6v or more mean they are probably fully charged. 12.1v or less, don’t use it, charge it. Ideally you never want to see it below 12.1v. You can measure the voltage with a multimeter - which is a really useful thing to have - but you could just get a voltmeter that plugs into the 12v socket for a quick check (I wouldn’t leave it plugged in though). There are fancy battery monitors which are usually more accurate, but I used this method for several years and my batteries were ok.

Battery capacity is measured in amp/hours (Ah). So if your battery is labelled 60Ah, you’re only going to get a useable 30Ah from it, but those are 12v amp/hours. Using your USB socket as an example 2.1 amps at 5v is roughly 1 amp at 12v, so you would get roughly 30 hours of device charging time.

All this is a simplification, and if your batteries aren’t new, you probably won’t get as much out of them as it says on the label.

When you are charging the batteries, whether with the engine or a battery charger, the voltage will go up to around 14.4v - this is normal. After stopping charging, the voltage will drop down to 12 point something after a while.

Charging batteries to 100% can take hours. Getting to about 80% (bulk stage) doesn’t take too long, but after that, the rate of charge that a battery can accept gradually decreases as you get closer to 100% (absorption stage). As an example, using a battery charger, my battery will take an hour from 65% to 80%, but then takes 5 hours to get to pretty much 100%.

I would definitely get some solar as soon as you can?.

And you should really have a fuse between the battery and the socket!

Hope this helps.

 

[AuntyRinum]

If all you are doing is running a USB charger then you don't have much to worry about. What is the combined Ah of your two batteries?
If you run your charger for an hour you will take 2.1 Amp hours out of your batteries. If you charge with it non stop for 24 hours, which seems unlikely, you will reduce your available battery Ah by 48.24 Amp hours (2.1 amps x 24 hours). To recharge the batteries to their previous state you will have to put 48.24 Amps x approx. 1.1 (110%) i.e. 53 Amp hours back into the batteries.
As long as your leisure batteries are large enough and your engine can supply that charge then you are OK.
In reality you are not likely to use your charger to that extent so it's probably a non problem.

 

[MarkCX]

If you run your charger for an hour you will take 2.1 Amp hours out of your batteries.

But those 2.1 amps are at 5v so it will be closer to 1 amp per hour allowing for a little loss converting between 12v & 5v.

 

[AuntyRinum]

But those 2.1 amps are at 5v so it will be closer to 1 amp per hour allowing for a little loss converting between 12v & 5v.

The Ah really depend on the amps that the device being charged draws.
The voltage is only the pressure. The 2.1 amps is the maximum potential current.

 

[MarkCX]

2.1A at 5v is 10.5W
10.5W is 0.87A at 12V
So my rough 1A with the assumptions I gave is pretty much correct.

2.1A from the charger for 24 hours will not consume 48Ah from the battery.

 

[Mistroma]

This would probably be better asked in the “reader to reader” forum, some really knowledgeable people there who don’t necessarily come into the “live aboard” forum, and are much better qualified than me!

As for basics, in simple terms, running your batteries down to below 50% will almost certainly kill them really quickly. A very rough and ready way to check is by measuring the battery voltage when they’re not powering anything. 12.6v or more mean they are probably fully charged. 12.1v or less, don’t use it, charge it. Ideally you never want to see it below 12.1v. You can measure the voltage with a multimeter - which is a really useful thing to have - but you could just get a voltmeter that plugs into the 12v socket for a quick check (I wouldn’t leave it plugged in though). There are fancy battery monitors which are usually more accurate, but I used this method for several years and my batteries were ok.

Battery capacity is measured in amp/hours (Ah). So if your battery is labelled 60Ah, you’re only going to get a useable 30Ah from it, but those are 12v amp/hours. Using your USB socket as an example 2.1 amps at 5v is roughly 1 amp at 12v, so you would get roughly 30 hours of device charging time.

All this is a simplification, and if your batteries aren’t new, you probably won’t get as much out of them as it says on the label.

When you are charging the batteries, whether with the engine or a battery charger, the voltage will go up to around 14.4v - this is normal. After stopping charging, the voltage will drop down to 12 point something after a while.

Charging batteries to 100% can take hours. Getting to about 80% (bulk stage) doesn’t take too long, but after that, the rate of charge that a battery can accept gradually decreases as you get closer to 100% (absorption stage). As an example, using a battery charger, my battery will take an hour from 65% to 80%, but then takes 5 hours to get to pretty much 100%.

I would definitely get some solar as soon as you can?.

And you should really have a fuse between the battery and the socket!

Hope this helps.

Excellent summary. If OP has no instruments, nav. lights, FM radio etc. then his requirements are pretty much the square root of sod all. He's at a level where other factors probably have a significant impact.

1) Battery capacity drops as temperature is reduced
2) Capacity increases as temperature is raised but battery lifespan is also reduced at higher temperatures
3) Battery capacity will be higher with very low current consumption e.g 100Ah battery might theoretically yield 20A for 5 hours but 1A for 130 hours (won't get either of course)
4) Capacity decreases with age so OP won't know actual capacity
5) Batteries self-discharge even when not being used for anything (Could be significant as OPs usage is so light)
etc.

Above items are largely of theoretical rather than practical interest, other than to demonstrate that OP would find it difficult to actually work out battery capacity.


It is worth trying to use the correct charging regime for his batteries and keep at 100% on float when possible. OP doesn't say if he sails weekends and leaves the boat on a mains charger during the week.

A small solar panel should solve all his problems and I really mean a pretty small, cheap rigid panel.

If OP arrives with a charged phone and leaves it charging for 24 hours he'll be lucky to use 8Ah from his 12V battery. A 50W panel will easily produce more than that on average during any day between April and September (Flat on deck, South coast UK). To be honest, a 20W panel would probably be able to keep a 100Ah battery at float during the week and above 90% most of the weekend.

OP won't need to spend much to keep a battery well charged unless he's planning to spend an awful lot of time living on his boat. The big unknown part will be "instruments and lighting". OP doesn't give any detail but use of either will probably have a larger impact than simply charging a phone, leading to an increase in panel size needed. More information is required on sailing plans and actual equipment fitted. LED lights are a no-brainer of course.

PS
I'd like to bet OP's actual power needs drift up pretty quickly as bits of kit arrive and become vital.

 

[Graham376]

OP won't need to spend much to keep a battery well charged unless he's planning to spend an awful lot of time living on his boat. The big unknown part will be "instruments and lighting". OP doesn't give any detail but use of either will probably have a larger impact than simply charging a phone, leading to an increase in panel size needed. More information is required on sailing plans and actual equipment fitted. LED lights are a no-brainer of course.

According to OP's post - As we are in the process of moving to live aboard her full time. We don't know whether marina or mooring based or whether separate engine and domestic batteries and of course what the liveaboard current draw will be. The 12v charger plug will be the least of his/her worries.

 

[Mistroma]

According to OP's post - As we are in the process of moving to live aboard her full time. We don't know whether marina or mooring based or whether separate engine and domestic batteries and of course what the liveaboard current draw will be. The 12v charger plug will be the least of his/her worries.

Wow, I must have been sleepier than I thought and completely missed that. I don't think I even realised that it was on liveaboard and probably assumed it was a new guy on PBO forum.

You are correct, phone charging won't even register as a minor blip for someone thinking about living on a 23' boat. I doubt he's anywhere close to realising the problems he needs to address, especially if UK based in winter. He could of course generate all his electrical power from a thermopile running off some Tea-lights, thereby solving power and heating needs at the same time. Probably about as much use as the advice I gave for someone planning to live aboard.

 

[Darwining]

(Wow, step away for a day and come back to a PLETHORA of replies! sorry if I don't respond to everyone!)

This would probably be better asked in the “reader to reader” forum, some really knowledgeable people there who don’t necessarily come into the “live aboard” forum, and are much better qualified than me!

As for basics, in simple terms, running your batteries down to below 50% will almost certainly kill them really quickly. A very rough and ready way to check is by measuring the battery voltage when they’re not powering anything. 12.6v or more mean they are probably fully charged. 12.1v or less, don’t use it, charge it. Ideally you never want to see it below 12.1v. You can measure the voltage with a multimeter - which is a really useful thing to have - but you could just get a voltmeter that plugs into the 12v socket for a quick check (I wouldn’t leave it plugged in though). There are fancy battery monitors which are usually more accurate, but I used this method for several years and my batteries were ok.

Battery capacity is measured in amp/hours (Ah). So if your battery is labelled 60Ah, you’re only going to get a useable 30Ah from it, but those are 12v amp/hours. Using your USB socket as an example 2.1 amps at 5v is roughly 1 amp at 12v, so you would get roughly 30 hours of device charging time.

All this is a simplification, and if your batteries aren’t new, you probably won’t get as much out of them as it says on the label.

When you are charging the batteries, whether with the engine or a battery charger, the voltage will go up to around 14.4v - this is normal. After stopping charging, the voltage will drop down to 12 point something after a while.

Charging batteries to 100% can take hours. Getting to about 80% (bulk stage) doesn’t take too long, but after that, the rate of charge that a battery can accept gradually decreases as you get closer to 100% (absorption stage). As an example, using a battery charger, my battery will take an hour from 65% to 80%, but then takes 5 hours to get to pretty much 100%.

I would definitely get some solar as soon as you can?.

And you should really have a fuse between the battery and the socket!

Hope this helps.

Excellent summary. If OP has no instruments, nav. lights, FM radio etc. then his requirements are pretty much the square root of sod all. He's at a level where other factors probably have a significant impact.

1) Battery capacity drops as temperature is reduced
2) Capacity increases as temperature is raised but battery lifespan is also reduced at higher temperatures
3) Battery capacity will be higher with very low current consumption e.g 100Ah battery might theoretically yield 20A for 5 hours but 1A for 130 hours (won't get either of course)
4) Capacity decreases with age so OP won't know actual capacity
5) Batteries self-discharge even when not being used for anything (Could be significant as OPs usage is so light)
etc.

Above items are largely of theoretical rather than practical interest, other than to demonstrate that OP would find it difficult to actually work out battery capacity.


It is worth trying to use the correct charging regime for his batteries and keep at 100% on float when possible. OP doesn't say if he sails weekends and leaves the boat on a mains charger during the week.

A small solar panel should solve all his problems and I really mean a pretty small, cheap rigid panel.

If OP arrives with a charged phone and leaves it charging for 24 hours he'll be lucky to use 8Ah from his 12V battery. A 50W panel will easily produce more than that on average during any day between April and September (Flat on deck, South coast UK). To be honest, a 20W panel would probably be able to keep a 100Ah battery at float during the week and above 90% most of the weekend.

OP won't need to spend much to keep a battery well charged unless he's planning to spend an awful lot of time living on his boat. The big unknown part will be "instruments and lighting". OP doesn't give any detail but use of either will probably have a larger impact than simply charging a phone, leading to an increase in panel size needed. More information is required on sailing plans and actual equipment fitted. LED lights are a no-brainer of course.

PS
I'd like to bet OP's actual power needs drift up pretty quickly as bits of kit arrive and become vital.

Thanks so much for all the info! I'm away from the boat at the moment, but I'll try and do a dive down into the electrical guts and the batteries themselves to see what I can learn and apply.

GOOD TO KNOW about power usage. We've been idly using one battery for lights/leisure, and one for running (starting engine, running instruments) Although at the moment, instrument wise we only have a dubiously functional depth sounder.... definitely going to look into nautical GPS phone solutions... We haven't had much time to gauge our exact AmpH on a given day, but I'm hoping to snag a multimeter relatively soon and take some measurements as we get adjusted to living.

Ultimately, I definitely want to rig up solar panel(s), but first we're looking for a berth to overwinter. So hopefully shore power/water/facilities, but I like to be prepared if we need to be Off Grid for any amount of time. Better safe than sorry!

Right now, we're berthed up against a wall, which is better than the tidal mooring we started at, but still no shore power.

 

[Mistroma]

I had dozed off, woke again and then checked new posts before getting back to sleep. I think I read a new post in PBO and then your post. The live on board bit completely escaped me.

Heating and ventilation are going to be much bigger problems than running a phone or tablet. I'm assuming you are in UK for sake of argument.

Boats tend to be poorly insulated and also lacking in decent ventilation. Heating water takes a HUGE amount of energy and heating air takes a fair bit (made worse by continually changing it for cold air). Don't waste time on schemes to heat water from a battery, you can't change the laws of physics. Burning fuel (and breathing) will produce condensation and require more ventilation. Electric heating won't produce moisture but needs mains power (forget a generator on a 23' boat). Don't consider using a gas cooker for heating, dangerous, inefficient and lots of condensation.

I'm guessing you might either go for a drip feed diesel heater or get shore power.

You really have solar or wind as main options for off-grid power. Wind will be expensive, noisy and actually pretty useless during most of the summer. Solar will be much cheaper, fine in the summer but it drops off a cliff in winter.

Estd. daily output from a 100W solar panel lying flat on deck with no shading (in Ah)
Jan 4
Feb 7
Mar 13
Apr 20
May 23
Jun 26
Jul 25
Aug 21
Sep 16
Oct 9
Nov 5
Dec 3

Output can be improved if the boat doesn't swing and panels can be tilted without shading issues. It won't make a huge difference in summer but can almost double output in December and January (only increases from 3-4Ah/day to 7Ah/day).

I don't think that you need to bother with a multi-meter initially as a spreadsheet will give a reasonable indication.

List every electrical item, wattage and how much use it gets each day. It's usually pretty easy to find out the wattage.

e.g.
Light bulb: 6W and used for 3 hours each evening on 12V system
Current required is W/V i.e. 0.5A (6W/12V)
Daily power use is therefore 1.5Ah (0.5A x 3 hours)

Repeat for all equipment using your best guess. Use a meter if required but it is a lot easier to just look for a label on each item. The biggest error is usually in the estimation of hours use per day.

Add everything and you get an idea of daily usage.

Cooling is power hungry and a fridge will be the largest power user if you have one. An insulated box in the cockpit works well in Scotland outside summer months (often mid-summer as well ).

 

[MarkCX]

With regard to heating, as Mistroma said, a drip feed diesel is ideal if you don’t have guaranteed mains. Some people might recommend a Chinese Eberspacher type diesel heater, I have one, it was cheap and it’s good, but it still requires some electricity to run the pump and fan, and solar on a 23 footer just won’t be enough now the days are getting shorter - I know from experience ?. Half an hour before getting up is an extravagance. After that it’s just layers - but I don’t mind roughing it.

Mistroma’s comment about an insulated box is a good one too. I’ve just finished making a custom insulated box. I had a shop-bought coolbox, but it was hopeless. I took it apart and the insulation was meagre to say the least. I’ve now got a good couple of inches of insulation and it’s working well.

And for lighting, if you haven’t done so already, swap out any old style incandescent lamps for LEDs. On a tight energy budget, even the smallest change is worthwhile.

 

[AuntyRinum]

But those 2.1 amps are at 5v so it will be closer to 1 amp per hour allowing for a little loss converting between 12v & 5v.

Amps drawn from the battery are amps drawn from the battery whether you draw them at 5v or 12v. However, as has been said, that will be a minor blip on the total draw from living aboard.

 

[MarkCX]

Amps drawn from the battery are amps drawn from the battery whether you draw them at 5v or 12v.

Roughly, to provide 2.1A at 5v the USB adapter takes 0.85A at 12v. It’s the same amount of Power.

And just to reassure myself that I haven’t lost the plot entirely, I’ve just replicated the scenario on my boat and measured everything to check that I am correct (I am)

[Sorry about the thread hijack Darwining. My limited powers of explanation are now exhausted and I’ll leave it at that]

 

[greeny]

"Hello! I bought a boat with a friend and we have very little idea what we're doing! A Hurley 24/70, with two 12v leisure batteries for electrical, powered solely by the diesel inboard engine (High hopes to kit her out for solar! Eventually...). Aside from instruments and lighting, a single 12v plug wired right into the batteries: no switch for the plug aside from the selector switch. "

Small boat with no "mod comforts" at the moment. Your biggest drain at the moment will be your lighting, (I see no mention of fridge).. Change the lamps for led. Then as you add things to improve your lifestyle you need to primarily look at low power devices to keep usage down, and also at means to keep the batteries topped up to compensate your increased usage. Solar may be the best option but that really depends where you are based. You don't get much from a solar panel in Shetland in winter. Wind generator would be the best option in that location.
Of course you can't beat a 16 amp socket on the dock along with a battery charging system if you're planning a marina berth for the winter.

 

[greeny]

Volts x Amps = Watts
Watts x Duration = Amphours

 

[MarkCX]

Watts x Duration ≠ AmpHours

Amps x Duration = AmpHours
1 amp x 12 volts = 12 watts
1 amp x 1 hour = 1 AmpHour

Ok, I was going to leave it, but now someone else is joining in I can’t help myself.

darwining is trying to learn about amps, volts, watts, amp hours etc. So this is too important for me to just drop.

 

[greeny]

Sorry - yes - brain going stupid. My mistake. I knew what i meant to type but fingers didn't do it. Thanks for the correction.

 

[MarkCX]

You’re welcome Greeny. Hope I didn’t come across as too grumpy! This is usually one of the ‘gentler’ forums on YBW!

 

[Darwining]

Golly, s'been awhile since I've checked in.

No worries about thread Hijacking, I'm very happy just to read and try and ingest as much knowledge as I can. Thanks VERY MUCH for the info about the cold's effect on the batteries, I tangentially knew that temperature had an effect on battery life but didn't even consider how dramatically it will affect ours. Luckily we're staying in a marina overwinter, so shore power and battery lights will keep most of the wear off of our main batteries.

In the meantime, I think I'll simply have to hunt down a multimeter and try and start applying some of this stuff, at least out of academic interest. Thus far we've been pretty good at staying frugal and keeping battery usage to a minimum, especially considering we've no demands regarding cooling/heating (yet at least, may consider that diesel heater in the dead cold of winter...), and even a few choicely placed AA powered lights have made our lighting needs pretty slim.

Cheers and thanks for humoring!
-D