Charge Computer with engine on or off?

[MarkJ]

Hi Folks

I am a layperson with things under the bonnet so please reply in words of 1 syllable

I have a very basic electrical set up (and thats the way it has to stay for a while). I do not have a regulator.

When I turn the engine on I get 13.3 to 13.6 volts.

If I turn the 12v fridge on, or the computer to charge, it drops down to 13.0 volts.

As I have AGM batteries that means the 'float' voltage is about 13.2v.

Its bad to go under the 'float' voltage isn't it?

So, am I better to run the engine to charge my house batteries, and then, when its off, the charge my computer and run the fridge?


Thanks for your help

Mark

[btrayfors]

Mark,

Really need to know a lot more about your system, including your comment "don't have a regulator". ?????

In general, it's perfectly OK to charge the computer and run the frig while your run the engine. Each of these will draw between about 4 and 6 amps @ 12 volts.

Not sure where or how you're measuring the voltage drop to 13.0 volts, but a slight voltage drop is normal when you turn on a load. Compensation back to the float voltage level should take place if you have a smart battery charger and/or a smart regulator while you're running the engine.

Bill

[RBEmerson]

Yep - what he said! Particularly the "do not have a regulator" part - is there a word missing somewhere?

[MarkJ]

Originally Posted by btrayfors

Mark,

Originally Posted by btrayfors

Compensation back to the float voltage level should take place if you have a smart battery charger and/or a smart regulator while you're running the engine.

Bill

Hi Bill,

Thanks for that

No I don't have a smart regulator etc. I do have whatever is basic and within the alternator, but its not a 3 step regulator or smart one.

At this stage, as I say, I only have what I have so need to make thigs as good as I can for the system I have got.

I use a multimeter to measure the voltage drop. I do this by sticking it in the cigarette lighter socket on my panel.

The voltage drop is not momentary, its stays steady at the lower voltage for the duration of the draw.

Yes, I realise my system isn't optimum.

So with the voltage down to 13.0 volts do you think is it still OK to run the computer and fridge while the engine is charging the house batteries?

Thanks for your help


Mark

[btrayfors]

Mark,

Sure. It is what it is

The 13.2V float level is recommended for long periods of time. For many flooded batteries, it is IMHO much too low. At normal temperatures (say, 70-80's F), 13.6 or 13.8 is even better.

Charging voltages, by contrast, need to be higher. Normally, for flooded batteries, you wanna see 14.4 or even 14.6 volts during the charging phases.

Batteries die mostly because of undercharging and failure to maintain a full charge.

Don't know your situation, but if it's possible to add a smart charger -- and one with pulse charging protocols like the Iota series -- then your batteries will love you for it.

If not, well, that's just the way it is and you have to live with whatever comes along!

Bill

[sabray]

It s fine as far as I can see.
You should and want to cycle the batteries a little.
The way I read the profile for lifeline agms Yours may be different
More then 5% discharge is better more then 50% is bad
So if you discharge the batteries down to say 12.55 volts by running the fridge or charging the laptop you should be okay. Then start recharging the batteries and bring them back up close to 100% with the designed bulk charge voltage. the float is more of a maintenance thing keeping them topped up. If you are hovering around the float your fine but I think you should be giving them a deeper cycle once and a bit.
Any way thats how Ive understood it I could be wrong.

[cal40john]

First, measure the voltage at the battery with your DVM. You could be seeing voltage drops in the wiring that won't accurately reflect what voltage the battery is seeing.

Second, what RPM are you running? Try varying the throttle while measuring the voltage at the battery with your loads on. Alternators do not put out full power at low RPMs.

If you have charged the battery at the higher voltages without the loads, then I wouldn't worry about whether the voltage was 13.2 or 13.0 while you're running the loads. Far more important is that you don't want the float voltage too high or you'll overcharge and damage the battery, which is probably not an issue in your case.

You do need to run part of the time at voltages higher than the float charge to completely charge the battery. Float voltage is what is used to maintain a charged battery in its charged state, for many, this is the voltage the battery is kept at with a shore powered battery charger while the boat is not being used.

John

[RBEmerson]

Particularly with AGM's, which are more sensitive to voltage issues, you really should go with a smart regulator set for AGM's. I understand the need to keep things simple but when you went with AGM's (as opposed to flooded cell batteries), you rather stepped out of the "keep it simple" box. Sorry, but that's the way it is.

Measure the battery voltage at the batteries (unless you want to wire up a set of test terminals with the proper heavy wire). Anything else is going to include any voltage drops in the wiring for your power point socket or whatever you use. In short, it won't be quite the real story.

[Chief Engineer]

Are you concerned about surges?

[Blue Stocking]

MarkJ,
Maybe talk to a good alternator shop tech and see what he/she says about converting your alternator to external regulator. With some models it's not a complex procedure. What say you, Chief/ btrayfors?

[senormechanico]

MarkJ,

You haven't mentioned if you're charging the computer directly from 12V or if it's going through an inverter. Actually, it doesn't make any difference as most if not all computers these days have all the regulating circuitry inside the computer's battery case. Take a look at the AC "brick" coming with any laptop these days and it says stuff like (output) 18VDC 4.0 A (or similar). The computer will do just fine all by itself.

If you are running the engine and using an inverter at the same time, (even a small one like 300 watts) the computer won't know the difference from shorepower and it will charge normally. This may be the best option.
Steve B.
(30 years professional electronic experience)

[MarkJ]

Thanks very much for everyones advice.

As usual with things I don't know and understand there are concepts that takes a while to permeate my brain!

For a start the batteries and system have been set up by the previous owners, Sunsail. And work fine. The batteries are new - or newish - but I dont know the exact date.

I can't go buy a smat charger as I don't have no meny and the girl needs to eat.

My house batteries are only 2 x 55ah so I need to be careful. I charge twice per day, 4 times per day while at sea for 30 minutes each time. That basically just runs the auto pilot and instruments.


The charging of the computer / refrig is just a new thing as we are being naughty and drinking cold beer and I've got a new wireless internet connection (count my posts!).

So I am trying to make do with very little.

I want to buy a 100ah battery to add to the 2 x 55's and put in a smart charger, but the cost of that, installed (I can't do the smart charger) will be about $1,000. So I am trying to make do till I find a bank to rob

I have checked the voltage drop in the cable between the batteries and the socket I measure from and its quite a small drop, and allowed for. I can't get to the battery bank at sea, so one must make do


Finally, thanks again Your thoughts and advice are very valuable to me!


Mark
PS I'm away for a day or 2 so I have more ponder time

[Ex-Calif]

Originally Posted by MarkJ

I want to buy a 100ah battery to add to the 2 x 55's and put in a smart charger, but the cost of that, installed (I can't do the smart charger) will be about $1,000. So I am trying to make do till I find a bank to rob

I have checked the voltage drop in the cable between the batteries and the socket I measure from and its quite a small drop, and allowed for. I can't get to the battery bank at sea, so one must make do

Mark - I don't want to be too simplistic as you probably know more than you think you know.

Adding more battery capacity simply means that you have longer between charges but without increased charging capacity (bigger alt, solar etc.)you also have more charging time.

Consider the battery bank as a reservoir. You now have 110 ah and you plan to add 100 for a total of 210. A rule of thumb is that you don't want to use more than 1/2 the available as going further shortens battery life. I personally think deep cycle batteries will go a lilttle further but stick with 1/2 for now. So, you currently have 55ah usable and plan to have about 105 usable.

Do a consumption plan and you can calculate how long in time the 105ah will last.

Recharging - In simple terms the internal regulator has a sense wire that senses system voltage. What is key is where the sense line picks up the signal. A short run to the battery will cause charge voltage to be lower. because there is less resistance in the system on this short run and the sense wire sees a higher voltage close to the alternator.

A sense line that runs to the bus may sense a lower system voltage as there is more resistance between the alternator and the sense location.

The ridiculous example is the sense wire attached to the output wire of the alternator, effectively sensing voltage as it comes out of the alternator - not useful. The other end of the spectrum is so much resistance that the sense voltage is always low, the alternator amps up to max and can cook the batteries. A failed sense wire (open) can cause an alternator runaway.

When you add load at the same time as you are charging the problem of resistance is increased. After the short run to the battery you may see decent charge voltage but when you test the bus voltage at the panel or beyond (lighter plug) it is lower (13.0 etc). This is because resistance goes up with current draw. Sensed close to the battery the system voltage will be higher and the internal regulator will be reading a higher voltage and dial back the charge rate(voltage).

This is why some old cars had dim headlights and so on. The alternator wasn't producing it's maximum because it was sensed near the battery. Resistance in old wires and connections (all the way to the headlights) dropped bus voltage even lower.

In your case I would suggest that the regulator sense is pretty close to the regulator - because that is usually standard. The cigarette lighter plug is out at the end of the circuit and votage will read lower. Especially during a period where the battery is charging heavily and/or you have load on.

We have a very simple 3 (green, yellow, red) light indicator that we plugged into a lighter socket. We monitor house until the green is off, the yellow is on and the red starts to flicker. Then we run the engine and the green light comes on immediately assuring us the system is charging.

Using accessories while charging. There is a limited amount of generation available. You probably have a 50 or 60 amp alternator. I would reckon about 60-70 percent effieciency and say that you could put in 40 amps per hour into the batteries. Your fridge is pulling about 48 amps per day and the other incidentals - unless you have heavy light usage - probably another 20.

Your two 30 minute charge periods is putting about 80 amps in and you are using 68. So you are balanced. If you add batteries and consume more you will have to add charge time to keep the reservoir full. If you have a total of 105 usable and you used it all you would have to up the charge time to maybe 1 1/2 hours a day.

Charging the computer while the engine is on is not problem in my opinion. We do it all the time. The inverter and the computer regulator will handle most votage variations.

[Hud3]

Mark,

I have Lifeline AGMs, and have had some chats with the tech guy at their headquarters. Among other tidbits, he told me that a battery bank should be made up of batteries of the same size and type, and the same age. Don't mix 'em up, or their lifespan will be shortened. Also, for AGMs, the charging voltages are critical. You really need a smart, three-stage charger, or you will shorten the lifespan of your bank. Until you can afford the smart charger, I'd hold off increasing the size of your bank.

[bobsadler]

dan: what's the simple indicator you have plugged into the cigarette lighter outlet

thanks