Practical Efficiency of Large Lithium Batteries

Hello Bill,

I'm at a loss to understand why you are concerned about the heat issue. In general, one should avoid charging Lithium batteries below 0°C because it promotes metal plating at the anode which can develop into an internal short, producing excess heat, shortening life, and potentially sets up an unstable and unsafe condition.

Heat is not a phase in the conversion of energy. Charging and discharging can cause heat in a variety of places. But high currents in either charge of discharge of a battery will shorten the life of the battery. So, heat is usually a secondary concern.

The significance of the waste of energy is arguable because the rate of charge or discharge determines this "waste". Considering the cost of these batteries, life is the primary factor. A proper design will take advantage of the properties of the life cycle and physical characteristics of the battery first. Consequently, the loss with respect to heat is a byproduct that is generally insignificant.

So if you are concerned that charging your electric car batteries will overheat your garage, you can relax. Your gas guzzler will probably do more heating of the garage as it cools off from your last trip out.

If you would like to read some more about this check out this rather cool web-sight for a snapshot look at the most likely future concerns. After driving 120,000 miles in this roadster you will need to replace the battery at cost of around $12,000.00. So, obviously battery life is a very significant concern. Don't sweat the heat!

Hello:

Thanks for your prompt reply, and your good information. My only interest is in an efficiency figure comparing the power expended recharging such a battery to the power recoverable upon discharging the battery. The fact that there is heat produced indicates to me that it must be less than 100%. I probably would guess something like 98% but that would be a pure guess. Maybe it is negligible. On the other hand, lithium batteries have been accused of burning vehicles.

I wish to make a statement comparing the efficiencies of cryogenic energy storage involving liquid air with lithium batteries, and need to know what figure to use for the lithium batteries.

Thanks; Bill Michaels

I am not sure about lithium batteries, but for lead acid batteries, the rule of thumb is you have to put in about 20% more than you take out.

This is a handy battery info.

Andy, sounds like you're an expert on LA batteries, what is the safe charging current level for them normally? Can it be > 10% or less only?

I am not an expert, I know enough to NOT damage an LA Battery when charging and I have built a dozen or so chargers over the last 9 years for myself and other caravan users. To date, no batteries have been damaged or needed replacement. The chargers I make cost less than most batteries to build.....

To answer your question:-

It depends upon the capacity of the battery. I go by the manufacturers info. If you cannot find any then go for a max current of 10% = 100 ampere/hours = 10 amps max....

It depends upon what you are doing and the type of LA battery also, but I charge from 12.6 volts to 13.2 volts only. This allows me to safely ignore the need for a temperature correction when charging.

The battery is then only about 70% full, but it never gases or gets undercharged. Damage starts below 12.6 volts. Gassing above 13.2.

Efficiency is around 80% then. A good figure for LA batteries....higher charging levels lower efficiency drastically.

Its better to buy a battery that is 40-60 % larger than needed so that you can stay in the efficiency band.....up to 70% charge.

I just wasn't sure about the safe charge level of other than the usual 10% even though I assumed the vehicle's charger might be pumping higher than that at times.

I saw some manufacturers denote the voltage level of a fully charged battery between 13.8-14.5volts however, as the battery ages these levels can drop under 13.5v even.

I read an article once that an LA cell's voltage level in discharged state is @2.1v but can be as high as 2.4v when it's fully charged.

Do you know a safe level of charging regardless of the battery's condition at which you can leave it on day and night, like through a solar panel?

What I told you before is always safe and is OK for any type of LA Battery - Gel, Leisure Car etc.....

Some batteries cannot give large currents for cranking - Leisure.

Some batteries cannot be allowed to discharge below 12.6 volts for long - car batteries

Its best to find tutorials on the web for the specific battery type you have to give you infos on the maximum charge current and voltage and at what temperature..... Temperature has a dramatic effect on high charging levels. How "full" you want the battery also plays a role.....

The levels that I recommended can be used without monitoring the battery temperature....or gassing or sulphation (which kills the battery dead!)......which is why I use them.....

"You pays your money and you makes your choice!"

Trickle, which for a 100 Amp/Hour battery would be around 500ma.....0r 0.5%C, where C=Capacity of battery in Amp/Hours.

that's another thing the trickle charge, i never new what % that is wrt to the battery's capacity.

0.5% is a good starting point, better is what the manufacturer says.

If you want to find out what the trickle charge value is, you need a stabilized variable voltage source (power supply) of say max 2 amps or so. Add a 5 amp diode to say the + connector to prevent back voltages into the power supply.

Measure the batteries voltage using a good digital voltmeter.

Connect the power supply and have a digital meter to read current.

Another to read voltage.

Set a current of say 500ma on the power supply.

Monitor voltage. If voltage climbs, no matter how slowly, the current is too high, reduce.

If the voltage drops over time, increase the current slightly.

If the voltage stays the same, you have found the trickle charge value for that battery at that time. (as the battery ages, this value will increase slightly.....)

Here is a simple diagram of how it needs to be connected:-

Are you suggesting that trickle charge is to replace the actual leakage current? That's what happens as the battery ages, it gets higher.

Thanx and in any case I gave you a big GA.

A good charger should re-charge the battery to a particular point, I like 13.2 volts personally, and then just hold the charge with a trickle...........

If you put in a higher trickle charge, eventually you will overcharge the battery and gas it to a degree......this will shorten the life of the battery.....

"the rule of thumb is you have to put in about 20% more than you take out."

This is a useful bit of information. thanks

bill michaels

Be aware however, that this rule of thumb applies to lead-acid batteries and not to the large lithium batteries you were asking about.

But not very accurate if you fully charge, its only accurate up to about 70% full....

Fully charged implies up to a 50% loss of efficiency.