Power Shortage

[Snboard976]

I can't figure out why I have such a large voltage drop when I use a laptop on board. I have a 500 ah battery back (less than a year old) and the only draw is two dell laptops that draw 1.5 amps each.

Both laptops are plugged into a 300 watt inverter. When they are both plugged in the voltage drops from 12.7 to 11.9 and the inverter sounds a very annoying alarm.

Can someone explain why this is happening, or what I could do to fix it?

[btrayfors]

Chad,

Need a bit more info. You say the Dell's draw 1.5 amps each. Is that 1.5 amps @ 120 volts AC?? If so, that's a lot for a laptop, and would translate into about 14 amps @ 12.6 volts DC. Accounting for inverter loss...another 15-20% most likely, or a draw of about 17 amps. Each.

If you meant 1.5 amps draw @ 12 volts, that's too little for a laptop.

Many laptops draw 6-7 amps @ 12 volts; some newer ones draw less.

So.... we need better figures on the laptop draw.

A healthy 500AH battery bank should be able to handle two laptops for awhile, but if it's capacity has been compromised -- thru undercharging/sulfation or overcharging/damage or other means -- then it wouldn't be able to supply the needed amps for very long.

A clamp-on DC ammeter would be very helpful here to see how much these puppies are really drawing.

Bill

[klem]

When you say the laptops draw 1.5A each, is that 120VAC? If so, the two laptops are maxing out your inverter. If you are actually drawing this much power, your inverter would be drawing over 30A @12VDC which will cause measurable voltage drop. Are you sure that the alarm means that the voltage is too low? Is it possible that it means that your draw is too large?

Where are you measuring the voltage? Are you measuring it at the battery or at the inverter? If it is at the inverter, you might well have a connection that isn't very good leading to this large voltage drop under load. Even if it is at your panel, poor connections between the battery and panel could lead to this large voltage drop.

[scottyt]

if they are drawing 1.5 amps each at 120 thats about 15 amps at 12 volts each, which i really doubt as 300 watts is only like 23 amps at 13 volts. now a ? is, is the voltage being shown on the inverter or a separate gauge/display? if its the inverter check the wiring size and connections to the inverter/battery.

some questions to get you going in the right direction

age of batteries ( answered in OP )
how are the connections
how cold are the batteries ( if in a bilge in cold water they can be quite cold even if the boat is warm )
what else is running at the same time
what is the true state of charge

i would lean to cold batteries or loose/under sized connections

[Snboard976]

The small box on the computer's charger says

Input: 120-240volts ~1.5A
Output: 19.5volts ~3.34 A

I wouldn't be suprised if it was on the high end of power consumption for a lap top.

It seems strange that the batteries can't keep up with the computers. After the inverter gives its alarm we shut everything off and go to sleep. In the morning the battery voltage is back up to 12.6 which I thought was fairly well charged.

[Snboard976]

The voltage shown is on a display located near my battery switch (away from the inverter.)

The alarm is definitely for a voltage drop. I get it when ever the voltage dips below 11.9 and the inverter is on, even with only one laptop plugged in. I don't get the alarm with both laptops plugged in when the batteries are charging.

[wind_magic]

It does seem strange. I am able to run a laptop from my battery bank for quite a few days without charging, I know because we had a big snow storm and it covered up the solar panels and didn't charge for days until I made time to clean the panels off, the voltage never did get that low.

Unrelated, have you considered using 12vdc instead of the inverter, you'd need a DC converter for that. That's how I do it, I don't know how much power it saves but I believe it saves a little bit.

[Snboard976]

It is definitely cold tonight and the leads to the inverter are about 15 feet long and only 10 gauge wire. Maybe I should look into an inverter that clips directly to the batteries.

[scottyt]

Originally Posted by Snboard976

It is definitely cold tonight and the leads to the inverter are about 15 feet long and only 10 gauge wire. Maybe I should look into an inverter that clips directly to the batteries.

okay the cold is good for the wires bad for the battery

10 gauge is rated for 30 amps, which if the power supply is right on its input amperage then yes you are over loading the inverter and then wires feeding it. if you increased the wire size or shorten the run you should be better, but look at a slightly larger inverter to give it some room to work with. remember if you are running the inverter at near max you are loosing a little extra in heat. ( not including the loses already with the inverter )

drop a thermometer in with the batteries to see how cold they are too, that could explain the drop and recover you are seeing. maybe even put a light bulb with the batteries if you have shore power. ( which it sounds like you dont )

okay for knowledge the voltage drop at 30 amps on 10 gauge at 14 volts on a 15 foot run is 6.7 % or about 1 volt in drop. if you are starting with 14 volts the inverter is only seeing 13 volts. at 12 volts its a 7.8 % drop or seeing 11

edit one laptop at 15 amps is dropping voltage about .5 volts on the wire run to the inverter

[SVPrairieRose]

I don't want to hijack your thread here but I have had similar problems with my last two laptops. I can run them from my inverters, one is a 150w and the other is a 300w, when the computer is fully charged. When the computer needs to be charged however, the inverters cannot keep up. Unless, and here's the rub, my engine is running or I have the charger plugged in. Even if my batteries are full or near full.