Which battery technology (Christmas is coming)

[jwhitmore]

Information on battery technologies is two a penny but some of it appears to be discredited, like the batteryuniversity website. (Great name for a website though).

What is important is, probably,  what I'm trying to do with a battery. I wanted to have a battery in a box with a solar panel for charging it. Then I want to use the stored power to power some small lights, on a tree.

I wanted to do this intelligently and that's the problem. I wanted to stick a uC in the box which can monitor the battery. Ideally a battery should have an LCD display which indicates the number of electrons in the battery. (Can't believe that's not been invented) So given that information the uC can intelligently decide whether it can afford to light the lights or not. So you'd have a high and low threshold for powering the load.
You might well say I'm complicating this but it's just a step you could get way more complicated then a Christmas tree load. You could have a Mars rover in the back garden, or an intelligent autonomous lawn mower and a charging station.
Actually the lawn mower is a good example as you want to monitor two batteries. Charging station and Rover.

So after all that intro Lead Acid are cheap as chips, well relatively, but really difficult to get this kind of information out of. Well from what I can gather. So you're probably going to end up with Lithium.

I was trying to look at "Off grid living" as I thought that books on that might be of assistance. So far not much, all very hippe and little engineering. So could anybody point me at engineering, that is creditable.
 

[Fungus]

You need something like this:

http://www.ebay.com/itm/221533778016

Combined with something like this:

http://www.ebay.com/itm/361120230414

(randomly chosen sellers)

nb. The charger has built-in protection. You don't need 'protected' batteries.

PS: Under no circumstances buy a battery with "fire" anywhere in the name.

[Siwastaja]

Ideally a battery should have an LCD display which indicates the number of electrons in the battery. (Can't believe that's not been invented)

This is how it's often done! It's called Coulomb counting. You do it by measuring current (using a low-value shunt resistor and an ADC) as accurately as possible, and integrate it over time. You need to reset the counting every now and then because of inaccuracies and leaks. Resetting is done at a known state, usually 100% full battery, which can be detected by the combination of the exact voltage and (low enough) current at that point -- for example, U = 4.20V and I = C/20. Laptop batteries usually do this. This works well with li-ion because of very little leakage current and very little side reactions that consume charge (coulombs), but not that well on most other battery technologies.

[calzap]

Ideally a battery should have an LCD display which indicates the number of electrons in the battery. (Can't believe that's not been invented)

This is how it's often done! It's called Coulomb counting. You do it by measuring current (using a low-value shunt resistor and an ADC) as accurately as possible, and integrate it over time. You need to reset the counting every now and then because of inaccuracies and leaks. Resetting is done at a known state, usually 100% full battery, which can be detected by the combination of the exact voltage and (low enough) current at that point -- for example, U = 4.20V and I = C/20. Laptop batteries usually do this. This works well with li-ion because of very little leakage current and very little side reactions that consume charge (coulombs), but not that well on most other battery technologies.

The familiar amp*hour is the transfer of a certain number of electrons.  An amp is one coulomb per second, and a coulomb is the charge of 6.242 x 10^18 electrons.  So an amp*hour is the transfer of 6.242 x 10^18 x 60 x 60 = 2.247 x 10^22 electrons.  One mA*hour  the transfer of 2.247 x 10^19  electrons.  Most folks prefer not to deal with such huge numbers, so A*hour and mA*hour are the common units.  kC (kilocoulomb) can be used.  A 5kC battery is a 1.4 A*hour battery, but keeping amps in the unit is useful for measurement and power calcs.

Mike in California

[jt]

If you buy a battery pack with built-in over voltage protection (pretty standard for any Li-ion battery pack) then you can connect the solar panel directly to the battery if you confirm that the solar panel can not produce more current than the battery can accept.  The conservative estimate here would be max rated power of the solar panel divided by the under-voltage threshold of the battery protection circuit.  This calculated value should be >> than the max charge current specification of the battery.  batteryspace.com has a wide verity of batteries and is generally good about specifying the information you will need. 

If you want to maximize efficiency you can dive into the world of solar cell peak power tracking, which would be a circuit you insert between the solar cell and the battery to deliver the maximum power achievable from the solar cell.  There are dedicated ASICs (Linear Tech and ST micro), or you can design your own hardware and algorithm with an MCU. 

Battery fuel gauging including counting electrons, (or coulomb counting) can get really complicated before it will give you consistently accurate results over time, temperature and cell ageing.  Luckily, many applications can get-by using battery voltage as an indicator of state of charge; it sounds like your application may fit in that bucket. 

As for controlling when your lights turn on and off, you can use a comparator with hysteresis.  As an example, if you have a single-cell Li-ion battery pack, you can expect the voltage to vary (nonlinearly) with state of charge between about 3.3V and 4.2V; so you would set your comparator to toggle on at 4.1V and toggle off at 3.4V (you can play with those values depending on your consideration of cycle-life vs. depth of discharge). 

Here is the first google result for more information on a comparator with hysteresis, but there are a lot of different ways to implement.
http://www.ti.com/lit/ug/tidu020a/tidu020a.pdf