New Battery - LiFePO4!

New Battery and Power Box

I usually image from places that have AC power--but not always.  When there isn't AC I need to run from batteries. It's gotten to the point that I like batteries so much I use them even when AC is available. But -- the batteries I've been using are heavy and bulky; they are really not fun to lug around. And they're barely adequate for multiple-night star parties, too.

My goal has always been to eventually give up lead-acid batteries for lithium. Lithium's advantages over lead are well known, but it has always been the price that stopped me from jumping on their bandwagon. This year that changed; prices came down a lot!

I sprang for a 50Ah LiFePO4 battery--1/3 the weight of my 50Ah lead battery, and that's a world of difference. The upgrade motivated me to put it into a smaller case that I had sitting around, and to improve the connection ports over those on my old case. 

The new case outputs are three 12V automotive sockets and two Anderson Powerpole (APP) connections. A third APP allows me to connect to my solar panel or an AC charger. There's an on/off rocker switch to toggle between charging and power box use, and a panel meter for monitoring the state of the battery (more about that later).

There's one change inside the box, too. All the outlets pass through an APP distributer that allows each to be independently fused using common bayonet-style fuses.

The new power box

Testing the new battery

Of fundamental importance with any battery is knowing when to stop using it. Discharging a rechargeable battery too deeply will reduce its capacity and useful lifespan. But what exactly is "too deeply" and how will we know when the discharge is approaching that?

Here is a much-copied table that you can find all over the Internet: 

Table relating State of Charge to resting voltage and
suggesting what is "too deeply" 

Some terminology:

• State of Charge (SoC) is basically a measure of how much energy is left in a battery compared to what it has when fully charged. A full battery has SoC = 100%, an empty battery has SoC = 0%.
• Operating Voltage (Vo) is what a voltmeter reads across the battery poles while it is in use. 
• Rest Voltage (Vr) is what that voltmeter would read when the battery is not discharging and has not been discharging for at least an hour. The table's voltages are all rest voltages. Vr is impossible to measure while imaging--unless you're willing to shut down to let the battery rest.

The third column in the table tells us SoC shouldn't slip past 20% if the battery is to stay nice and healthy. My battery is advertised to endure 4000 recharge cycles before needing replacement -- if the recharge comes before the SoC slips to 50%. Given my current age and how often I image, the battery will never see 4000 recharges; in fact I'd be surprised if it sees more than 200, and the majority of those will be from a SoC above 50%. But it's nice to know that if circumstances require it I can take it deeper. 

To be conservative about this, I'll preselect SoC = 20% as a safe stopping point. I'll probably never reach that because I can always recharge the next day using AC or my solar panel. 

If I can't recharge the next day and want to image anyway, how do I track the SoC so that I can prevent the battery from going too low? First, I'll need to know its energy content when the SoC is 100%. If I can determine how much energy it has provided since the last charge I can estimate SoC by

    SoC = 100% * (1 - (energy discharged / energy content at SoC 100%))

Given the battery's rated voltage and amp-hour capacity I can estimate the battery's full capacity:

   Energy content at SoC 100% = (rated V) x (rated amp-hour capacity) 

        = 12.8V x 50Ah 

        = 640 watt hours (Wh)

Why do I call this an estimate? Every battery is a little different even when new, and over time some of that capacity will be lost. Generally new batteries have a slightly larger capacity than their rating, but I'll go with 640Wh to be conservative.

I added a panel meter to the new box that gives me the amount of energy discharged; this makes  estimating SoC easy.

Panel meter showing a battery at rest after discharging 416Wh

If my aim is to not let the battery fall below a SoC of 20%, all I need do is make sure that number never goes over (1 - 20%) * 640Wh = 512Wh. That way, happy battery and happy imager!

Now you're probably wondering about that solar panel and asking if that isn't heavy and bulky, too. Oh, it is! But combined with the very light lithium battery it still beats the multiple lead batteries required for that rarest of events, a five clear night remote star party!

One more thing, a tiny hack. The panel meter has a button that lets you zero the energy discharge counter after a charge. It's also what turns on and off the panel backlighting.  The button is countersunk and can only be depressed by something relatively pointy. My solution is a 3mm glass bead held in the button hole by stretchy friction tape. You can see it just to the right of the panel display.

That's more than enough battery talk. Next time some even more mundane chatter about upgrading my Pegasus FocusCube to Version 3, and putting my old Version 2 on an AT-65.