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 Battery Power for Utilite and Utilite2 
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Joined: Mon Jan 09, 2012 3:16 am
Posts: 351
Post Battery Power for Utilite and Utilite2
Overview

Recently, compact, high capacity, high energy density lithium polymer battery packs designed for jump starting automobiles have become available. Many of these packs include 5V USB and 12V accessory power outlets in addition to the jump starter output. Such packs can act as a handy battery source for powering Utilite and a monitor in a remote application.

Here is such an application.


Hardware

Battery
Attachment:
Battery.JPG
Battery.JPG [ 245.4 KiB | Viewed 3137 times ]


The battery I am using is marked as a 10 amp-hour (40 watt-hour) type. In addition to the jump starter outlet, it includes a USB charging connector and a 12V accessory output rated for 10 amps. This makes it ideal for powering Utilite and a small, low power monitor.

See note on battery capacity at end of post.


Cable
Attachment:
Cable_Annotated.JPG
Cable_Annotated.JPG [ 39.09 KiB | Viewed 3135 times ]


In order to interconnect the battery, Utilite, and the monitor, I opted to construct a custom splitter cable. This cable is shown in the above picture.


Assembled System
Attachment:
Assembly.jpg
Assembly.jpg [ 245.57 KiB | Viewed 3135 times ]


The above picture shows the assembled system. Although the cabling arrangement isn't neat, it is functional.


Results

Both Utilite and the monitor work fine when running on battery power. I expect a full battery charge should provide around 3 hours of continuous use. I will test the actual longevity when time allows.

One drawback is that the battery pack I am using does not output power while it is being charged. This means it will be necessary to shut the system down once the battery is exhausted. With proper planning this should be a manageable shortcoming.


Last edited by hassellbear on Thu Feb 26, 2015 4:21 am, edited 3 times in total.



Wed Feb 25, 2015 5:54 am
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Joined: Mon Jan 09, 2012 3:16 am
Posts: 351
Post Re: Battery Power for Utilite and Utilite2
A Note About LiPo Battery Pack Capacity

You will notice the nameplate capacity of the battery pack I am using lists the unit's capacity as 10,000 milliamp-hours (10 amp-hours) and 40 watt-hours. At first glance, the unit's milliamp-hour rating looks impressive. However, further review shows the milliamp-hour rating is totally inconsistent with the watt-hour rating.

Based on the output voltages I have measured, I believe the pack utilizes 3 - 3.7V cells connected in series. This gives a nominal output voltage of 11.1V and a fully charged value of 12.6V (4.2V per cell @ full charge). An 11.1V pack with a 10 amp-hour rating should actually have a watt-hour rating of 111.

Watt-hour rating = Amp-hour rating X Nominal Pack Voltage

Watt-hour rating = 10 Amp-hour X 11.1V = 111 Volt-Amp-hours = 111 Watt-hours

However, my pack only lists a 40 watt-hour rating. How can this be?

Well, it seems some manufacturers of battery packs sum the amp-hour capacities of all the individual cells in the pack and list that value on the product. Unfortunately, that method is only valid if the individual cells are connected in parallel. However, the cells in my pack are actually connected in series, so the pack's true amp-hour rating at 11.1V is limited to the amp-hour rating of an individual cell. Thus, based on my pack containing 3 cells, the pack's amp-hour rating at 11.1VDC is 3,333 milliamp-hours (10,000 milliamp-hours divided by 3 cells). If you convert this new amp-hour rating to watt hours you get the following.

Pack Watt-hour rating = Cell Amp-hour rating X Nominal Pack Voltage

Pack Watt-hour rating = 3.3 amp-hour X 11.1V = 36.6 volt-amp-hour = 36.6 watt-hour.


... Which is fairly close to the nameplate value of 40 watt-hours.

The watch phrase here is Caveat Emptor. If you do a web search for Lithium Polymer Jump Starters, you will notice most of them list very high amp-hour capacities. It is my belief many of those ratings do not represent the pack's actual operational capacity.

What really counts is the pack's Watt-hour rating. Manufacturer's are required to provide the watt-hour rating due to international regulations governing the transportation of LiPo batteries, so I would trust that value more than the amp-hour rating. With regards to air transport, the watt-hour rating is what matters and what governs how the pack can be transported - checked baggage, carry-on, or not at all if it is over a certain size.

The bottom line is, don't base estimates of how long your Utilite will run on a pack using the amp-hour rating. Use the watt-hour rating instead.


A Battery Picture Is Worth A Thousand Words
Attachment:
Inside1.JPG
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Attachment:
Inside2.JPG
Inside2.JPG [ 81.05 KiB | Viewed 3102 times ]



My curiosity finally got the best of me, so I cracked open the battery pack to see what was inside. I was correct in assuming the cell stack was composed of 3 cells - the 3S (3 cells in series) marking confirms this. However, I was off just a bit on my assumption of 3,300 millamp-hours for the pack's capacity. The cell stack is actually rated 3,600 milliamp-hours which is equivalent to 40 watt-hours at 11.1V.

What is quite obvious is the pack definitely does not have a 10,000 milliamp-hour rating. The cell stack markings confirm it is best to trust and use the pack's watt-hour rating and not the inflated millamp-hour rating.


Thu Feb 26, 2015 4:01 am
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Joined: Thu Sep 25, 2014 9:43 am
Posts: 37
Post Re: Battery Power for Utilite and Utilite2
Hi,
I'm using a 4 cell LiPo battery providing 14.8V power. You can buy one in the closest rc model shop, it's not very expensive.
The Utilite can be powered with 10-16V power. As the maximum charge of this battery is 16.8V, it's safe to put a 12V voltage regulator after the battery.
I think that it might be a better idea as the Utilite becomes unstable when the voltage drops under 10V.

BTW, that could explain why you get less power than noted on the pack. Anyway, it's dangerous to deplete a LiPo cell under 3V. So if you are using just a LiPo battery, check regularily the voltage level of the cells.


Tue Mar 03, 2015 10:53 am
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Joined: Mon Jan 09, 2012 3:16 am
Posts: 351
Post Re: Battery Power for Utilite and Utilite2
Yes, a 4S LiPo pack would better match Utilite's input voltage range.

My 3S pack will probably reach Utilite's cutoff voltage (3.33V per cell) when it has reached 20% capacity remaining (80% discharged). That means Utilite will only be able to use 32 watt-hours of the pack's 40 watt-hour rating. Fortunately, the system should self limit before the pack is damaged.

Attachment:
LiPo Discharge Profile.gif
LiPo Discharge Profile.gif [ 28.38 KiB | Viewed 2986 times ]


Actually, LiPo packs are not well suited for a standby role such as an automotive jump starter where they are stored fully charged. In one year, a fully charged LiPo stored at room temperature will lose 20% of its capacity. In a potentially hot automobile interior, it may lose 35 - 40% of capacity in a year. When you need it most, it may not start your car.

It is for this reason that I store the LiPo batteries for my RC gear in a refrigerator at 40 - 50% charge. This reduces their annual capacity loss to around 2 - 3%. I only top them off just before I fly. This isn't very practical for a jump starter/computer battery.


Wed Mar 04, 2015 3:39 am
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