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Portable lithium power for camping

April 08, 2020 — BarryK

Unable to go camping, but putting home-isolation to good use -- starting a new project, a portable lithium battery box for camping.

My solar water distiller project isn't finished -- I had hoped that it would be, however, the last prototype did not perform as well as I expected. Sometime, I intend to modify the last prototype, to see if the efficiency can be improved. for now though, starting a brand new project...

I own a Dometic 45AH battery box, with deep-cycle AGM battery. This is lead-acid technology, and has various limitations:

EDIT 2020-05-09: Added item-7 to comparison list 

  1. Should only be discharged to 50%, any more will dramatically shorten the life of the battery.
  2. The battery must not be left in a partially-discharged state, it must be charged promptly.
  3. The number of discharge-recharge cycles is about 600-1000 (when discharged to 50%).
  4. Self-discharge about 2%/month at 20degC, 4%/month at 40degC.
  5. Lead-acid batteries are very heavy -- a 12V, 100AH AGM battery weighs about 29kg.
  6. Voltage drops considerably at lower state-of-charge.
  7. Discharge/recharge cycle 70-85% efficient (15-30% lost as heat). 

...they are just the ones that I can think if right now!

If the battery can only be discharged to 50%, that means my 45AH battery is effectively only 22.5AH. As I go camping infrequently, I need to be mindful of the self-discharge, and attach a battery charger every now and again.

Comparing the above items with a LiFePO4 lithium battery:

  1. Should be discharged no lower than 20%, otherwise life will be shortened.
  2. OK to leave indefinitely in a partially-charged state.
  3. The number of discharge-recharge cycles is about 2500 (when discharged to 20% capacity, 80% DoD), and about 5000 cycles when discharged to 50% capacity.
  4. Self-discharge is about 1-5%/month (I don't have any accurate figures for this).
  5. A 12V 100AH battery weighs about 11.5kg.
  6. Voltage stays fairly constant right down to nearly 0% capacity.
  7. Discharge/recharge cycle 90-92% efficient. 

Several years ago I bought a 110AH AGM deep cycle battery, but at over 30kg I could not lift it. Well, I could, at great risk of putting my back out. Sometime after that I bought the Dometic 45AH box --mostly because I could lift it!

So, finally have decided to go over to a lithium system for camping. I purchased an Amptron 12V 50AH battery -- oh joy, it weigh only 7kg! The company claims that even if discharged 100% (DoD), that is, down to 0% capacity, it will still give 2000 cycles -- hmmm. If I discharge it by 80% DoD (down to 20% capacity), that gives me 40AH, almost twice that of my Dometic battery box.

Here is the company web page for this battery:

...and immediately you will see a problem: the price. yes, AU$545, plus postage. For 100AH, you would be looking at around AU$800 - 900. You can find cheaper on eBay, but beware, they are cheap for a reason.

Anyway, I have been squirrelling-away parts required for this project over the past several months. Have recently ordered a couple of final parts off eBay -- coming from the other side of Australia, so may take awhile. Just about have everything ready to go, sitting here in my loungeroom, and can now eye-ball it all and visualize how it will fit together.

What I am proposing is a portable box, that will sit in the front of my vehicle, where the passenger's legs would normally go -- I am hoping that there will be enough space for the occasional passenger to place their legs beside/behind/in-front of it -- whatever.

But it could go anywhere, for example behind the front seats -- you would just need to run longer cables from the car's battery.

To get the ball rolling for this project, I need a circuit diagram, but I want one that also indicates the physical layout. I constructed the diagram in Dia, an app in EasyOS and EasyPup. This is the result, exported to PNG:


Here is the Dia file:


Here is the file exported to SVG:


EDIT 2020-04-12:
The circuit diagram has been improved, see later post: 

...more fuses, to make it safer.

A note about Dia: I found it to be adequate. I couldn't see how to create arbitrary snap-to nodes. There doesn't seem to be any object rotation. These are features that were in diagramming apps that I used 20 years ago in Windows. Anyway, managed to construct a reasonable-looking diagram.

The left side of the diagram is the "front", which would be the side facing the door, so that the readouts can be seen. The right side is the "back", furthest away from the door.

In a vehicle, you would bring a single-core cable from the ignition key. This will have 12V on it when the ignition is turned on, and tells the DC-DC charger that charging will be coming from the car's alternator.

A twin-core cable will come from the car's battery, to a red Anderson plug. Both of these can be easily unplugged if it is desired to remove the battery box from the car.

The circuit breakers are there just-in-case. but probably useful to turn off if not going camping for awhile -- the DC-DC charger does draw a small current in standby mode.

The Amptron coulometer LCD display also draws a small standby current, however I have wired that directly to the battery terminals, as it needs to be calibrated for the battery, and calibration might be lost if power is disconnected.

The black Anderson plug on the "front" is DC output, for whatever purpose required. It can also be used for charging the battery -- I have an Amptron mains-powered 15A LiFePO4 battery charger, which can plug into the black Anderson plug -- handy to recharge the battery if it is not being used for camping, maybe removed from the car. 

That's enough for this blog post. I plan to provide more details as the project progresses, such as where to buy the various components, design of the box, putting it together, and anything else required. 

Tags: nomad