Deep Cycle battery and usage question

kcmc68

Hi everyone,

I have a pilot project and am thinking of running 2 laptops off a deep cycle battery to see how it goes.

Here are the specs:
Laptop 1: 3.42 A, 65 W, + 19V
Laptop 2: 3.25 A, 65 W, 20 V
Deep Cycle Battery Total Capacity: 120 Ah

I read that we can only use 50% of the total battery capacity.

a) If I were to power both laptops on for 10 hours a day, would 50% of the total battery capacity be enough to last? If not, what percentage of the battery capacity would allow me to do this?

b) If I use regular household AC power to charge the battery, how long does it take to fully charge the deep cycle battery? I am using a 10 AMP battery charger (old).

Thanks everyone.

Cariboocoot

Re: Deep Cycle battery and usage question

Those laptop specs are maximum consumption numbers. They don't mean much for real-world use. You should get a Kill-A-Watt and see what they really draw on AC, especially in terms of Watt hours during a normal day's usage.

Now, there are ways of running some laptops directly off 12 VDC (various adapters for cars for example) but obviously a 20 Volt unit can't be powered by only 12 Volts.

That said, a 120 Amp hour 12 Volt battery can deliver a maximum of about 720 Watt hours DC. If you run the computers through an inverter, that inverter will consume some of the power and will have conversion efficiency losses as well. A good set-up might be 10% conversion loss and another 6 Watts for the inverter. Using the numbers you have got, one of those laptops would last for a bit over 10 hours maximum (720 Watt hours / 65 Watts = 11 hours run time approximately).

Again you'd have to have some actual consumption numbers to know whether or not it would actually work.

How long it takes to recharge the battery will depend on the depth of discharge, the rate of recharge (which is not a steady maximum) and the efficiency of the battery. There are also issues with the charging profile, as most battery chargers are not designed to do the Bulk/Absorb/Float cycles that deep cycle batteries need.

vtmaps

Re: Deep Cycle battery and usage question

kcmc68 wrote: »

I have a pilot project and am thinking of running 2 laptops off a deep cycle battery to see how it goes.

Why would you do that? You are on the grid and your laptops have their own batteries built in. If you are trying to build an UPS, then your deep cycle battery will be fully charged (floating) most of the time. You should buy a battery that is optimized for that type of service.

At the risk of derailing this thread, I can't help but comment on this response:

Cariboocoot wrote:

most battery chargers are not designed to do the Bulk/Absorb/Float cycles that deep cycle batteries need

I'm having trouble with the word "need". Most (but not all) deep cycle battery manufacturers recommend charging profiles that are NOT the Bulk/Absorb/Float cycles that are common in the renewable energy (RE) industry. Most deep cycle batteries are used in golf carts, forklifts, or UPS systems and are recharged on the grid. Their manufacturers recommend charging profiles that cannot be achieved by solar charging. The RE charging profiles allow for passing clouds... the grid chargers just assume that there is unlimited, uninterrupted power. Most deep cycle batteries can, in fact, be charged adequately by a RE Bulk/Absorb/Float profile, but I'm not sure that is what they need.

kcmc68 wrote: »

I am using a 10 AMP battery charger (old).

If it is an automotive type charger, it is may not be appropriate for deep cycle batteries. and among deep cycle batteries, there are batteries designed to be frequently cycled, and batteries designed to be mostly floating. You do need to find a charger that has a charging profile appropriate for your batteries. Depending on your batteries, it may or may not be a RE type Bulk/Absorb/Float profile.

--vtMaps

niel

Re: Deep Cycle battery and usage question

might it not be easier to buy an extra battery for each laptop and just put the 1st battery charged for each laptop aside in reserve while you use the 2nd battery after it was fully charged? it would be easier and maybe cheaper. you will need to remember to alternate using the batteries as they shouldn't sit in reserve for long time periods.

Cariboocoot

Re: Deep Cycle battery and usage question

vtMaps;

I always think long-term investment. Of course you can give batteries less-than-ideal recharge and then replace them sooner. Many people, in fact, do. Just usually not on purpose.

kcmc68

Re: Deep Cycle battery and usage question

So what charger should I use to charge deep cycle batteries? Yes, my old charger is an automotive one. Can I use a speed charger? Some guy on youtube has one that you can set how fast it charges the battery.

vtmaps

Re: Deep Cycle battery and usage question

kcmc68 wrote: »

So what charger should I use to charge deep cycle batteries?

Depends on the battery. The battery depends on the loads and the purpose of your system.

I think Niel's suggestion makes the most sense, but perhaps we don't understand what you are trying to accomplish.

--vtMaps

BB.

Re: Deep Cycle battery and usage question

Oh boy--AC Battery chargers are one of the industries that have not kept up with technology.

It is difficult to find high efficiency/programmable charge AC Charge controllers for off grid/small system use. The best options out there are usually some of the better Inverter/Chargers (Magnum, Outback, etc.) -- They have some pretty nice programmable settings for the AC charger sections of the inverters.

Xantrex (now Schneider) has a TC series of charge controller that looks very nice and older versions did work well (from what I understand)--However, they have been hard to get/marketing-supplier confusion about what is out three--Not sure where they stand today.

This thread goes way into to detail on smaller AC Battery chargers and smaller AC backup gensets:

Question about battery charger selection with EU2000 generator.

-Bill

Cariboocoot

Re: Deep Cycle battery and usage question

kcmc68 wrote: »

Some guy on youtube has one that you can set how fast it charges the battery.

Sure; you just keep cranking up the Voltage potential and the current goes higher and higher and charges the battery faster and faster.

Until you hit that point that the battery won't take and it burns up. Really not a good idea.

I agree with Bill that even the best AC chargers aren't really designed for RE use. There are some that come close. Examples: http://www.solar-electric.com/bach2.html But none are perfect, including those built in to some inverters.

techntrek

Re: Deep Cycle battery and usage question

kcmc68 wrote: »

Hi everyone,

...If I use regular household AC power to charge the battery...

There are many 3-stage AC chargers on the market now which will do fine with deep-cycle batts, no need to overthink this. You can even get them at Wal-Mart.

kcmc68

Re: Deep Cycle battery and usage question

Thanks all. The reason I ask about AC charger for deep cycle battery is that I plan to run a small solar system with it. If there's not enough sunlight outside for 3 to 5 days, I can plug the deep cycle battery directly into a household AC outlet to charge the battery to prevent the battery capacity from deteriorating.

If the charger from Wal-Mart supports deep cycle battery charging, can I just purchase that charger and it will work?

westend

Re: Deep Cycle battery and usage question

Yes, you can make it work but it is going to have some costs. As Cariboocoot posted, knowing the actual draw from the laptops is key. A Kill-A-Watt device can help you with that. With the usage numbers you can size the battery bank, taking into account the losses from inversion to AC and wire resistances. You would then size the modules to the charging needed to maintain state of charge for your batteries. A solar charge controller will be needed and one that is programmable is a good thing. You can then set your charging profile to match your battery type and make. If you can't get enough sunlight from your existing modules and need to occasionally charge the batteries, Black and Decker make a three stage portable charger that can be set for different battery types.
If you haven't purchased the battery, I would suggest buying 2 6v golf cart batteries and wiring them in series for 12v. It is about the cheapest, most durable configuration for small DC use.
Good luck with your system!