Is this normal

[Pollik]

Wilding and running off 2x75Ah batteries (installed together three months ago).

I am a heavy laptop user and it is a big laptop...the charger is rated at 75W, but generally runs at 60W or less (it runs off a 12v circuit fused at 5A).

It will run perfectly well for three hours and the battery meter- is well into the green. Within 30 mins, say, the charger will stop working (fair enough) but the meter has plummeted deep into the red...the charger ceasing to work is sudden (I guess it needs a certain voltage and is designed to stop below a cut off point), the meter changes quickly (but I have never sat and watched, waiting for it to fall).

It seems like odd behaviour. Is this a battery thing or is it just rubbish meters that are installed in MHs?




Polly

 

[Firefox]

I am surprised you run a 75W power supply off a 5A fused circuit.

75W at 12v is 6.25A. You said it takes less maybe 60W, but that's still 5A. Your fuse may blow at some stage.

Anyway, I'd expect you could draw 6A off a 150Ah set up for nearer 9-12 hours before the voltage starts to drop too much. Not advisable to take more than about 50% of your battery capacity or you will reduce your battery life. But it looks as though your batteries are not giving you 150Ah or your charger is very sensitive on input voltage. Continuous current draw at 5-6 A is actually quite a lot, it's like running a small fridge and we all know they can flatten the average MH leisure set up in one or two days. But I'd certainly expect more than 3 hours in any case.

First I'd invest in an accurate digital multimeter to get an accurate picture of your discharge curve - those red and green things are not too accurate. A digital multimeter will give you an exact figure. They cost only £10-12 at maplins,

Then I'd consider fitting at least 200-300 Ah of batteries if you are going to draw 5-6 A for any lenth of time. Also consider getting a lower powered lap top. I think both my TV and lap top are only 20-30 W each which means I can use them 2 or 3 times as long as yours. I don't usually have any problem for 2-3 + days on a 220 Ah set up.

 

[MikeH]

I have 2x75ah in my camper with tv and playstation, fluorescent light, cd/radio and can easily go two weeks of 3-4 hours an evening use. I have a 20amp split charge circuit plus a 1.5amp solar panel. There is no onboard charger from an ehu. I give the batteries a service charge from home every year before the summer and it never goes flat.

Whats the wattage of your laptop? Were your batteries fully charged before you started? I suspect not.

 

[Pollik]

I am surprised you run a 75W power supply off a 5A fused circuit.

75W at 12v is 6.25A. You said it takes less maybe 60W, but that's still 5A. Your fuse may blow at some stage.

It does, sometimes, if I am not careful.

First I'd invest in an accurate digital multimeter to get an accurate picture of your discharge curve

I have a multimeter...how would I plot this? It is because I am not getting as long as I expexted that I am trying to find out what is going on.

Another factor is the charging rate. I have only my alternator, rated at 700W as a power source. (Think I wil have to add solar power). Most of my driving is between 40 & 50 mph, so I figure I may be getting as low as 300W. This past few weeks, I have been in an area where daytime lights are compulsory, plus the fridge, plus recharging my partner's net book....all this might only leave 100W for recharging the batteries....that would take 3 hours from flat to full Factor in some res or shopping stos, say 5 or 6 engine startups.....only driving for a couple of hours. It is all finely balanced.

@Mike Yes, the laptop was fully charged to start with.


Polly

 

[Deleted member 3011]

If you have no other charging method except the engine alternator, and you don't spend 8 hours a day driving, it is quite likely that your batteries are nowhere near fully charged when you pull up to camp. 75% charged would be realistic and if you drive for only a hour, then even that would be highly optimistic.

Only way to know what is going on is to fit an ammeter in the circuit (or spend a lot of money on a battery monitor) but especially if your wiring to the battery is a bit light and the route length more than a few feet, you would typically find that the current into the leisure battery might start off at around 30 amps but quickly drop off to less than half that within 30 minutes and keep dropping thereafter. Work out the Amp-hours and the end result is a battery that is getting more and more depleted as the days pass.

A vehicle alternator is generally NOT a good charger for reasonably discharged leisure batteries - mainly because it is designed to keep a start battery charged just enough to start the engine next time.

 

[Squibby]

Have you ran the battery check on your laptop, if you have been using it for some time i.e a few years the laptop battery my be shot. the best policy with laptops is to remove the battery once it is fully charged and just run the laptop off the mains, this will prolong your laptop battery life and only insert the battery when you need to use it away from a power supply, as they tend not to last as long when your constanltly topping them up. This will also reduce your draw from the vans batteries as you will just be running the laptop and not recharging the laptop constanly.

Basicly don't charge something just for the sake of it. Pop out the fully charged battery, also do the same with the notebook you mentioned.

Give it a go and you should notice the differance straight away, also you will prolong your laptop battery life and leisure batteries life and save money in the long run as those laptop batteries and leisure batteries are not cheep.

Squibby.

 

[Pollik]

Thanx Tony

So, part of what you are saying is that part of my problem lies in the nature of the way batteries behave? 30 amps at 12v is 360W...which is more than my alternator will give under current drving condtions. 150 Ah of batteries, at 30A...5 hours.

Time for me to go away and play with the figures again, so I can understand what is going on. Working in Wh, my calculations showed batteries would recharge in around three hours. But thinking in Ah, per your post, means that recharge will take much longer. I don't quite have an understanding of the electrics here.

I will be back when I can find a hot spot...






Poly

 

[Pollik]

@Squibby No...the laptop is just weeks old. Ditto the netbook

 

[MikeH]

@Mike Yes, the laptop was fully charged to start with.

No, I meant the leisure batteries not fully charged. Tony has covered what I meant.

 

[Firefox]

I find my alternator does not give a very good charge even on a longer run.

Probably only 70-80% of capacity. So going down from 75% to 50% I'm only using about 25% of my battery capacity or 60Ah.

My electronic charger off 240v gives a much better charge more like 95% I think, so then I get 45% of 220Ah or 100 Ah from my batteries.

I guess you could get a better charge from your alternator using one of those Stirling charging systems or similar, but they are quite expensive.

In answer to your question about discharging, measure your battery voltage after a charging run with the multimeter. Then plug in your laptop and record the voltage every half hour till your lap top charger cuts out. Unplug the lap top each time you measure and let the batteries recover for a minute so you get the "unloaded voltage".

You'll get a graph with time versus voltage. You can compare the voltages against what a fully/partially charged battery is supposed to give. For example:

At 77 degrees F

100% charge - 12.63V
75% charge - 12.30V
50% charge - 12.00V
25% charge - 11.76V
0% charge - 11.64V

(see also Lead-Acid batteries )

You can see fully charged is only 12.63 volts. Now, say your alternator was giving you only 75% charge or 12.3 volts. And your laptop power supply was a bit fussy and needed 12.2 volts (60% charge) you are only using 15% of your battery capacity before the power supply cuts out.

15% of 150Ah is 22.5 Ah. So in this scenario you are are only going to be able to supply 6A for 3.5 hours or so (= 21 Ah)

You basically got stuffed by poor charging off the alternator, a fussy, power hungry lap top supply, and low battery capacity. Change any of these parameters and you get longer usage time.