Heat dissipation from battery's.

[hollandhvac]

Hello everybody, I am here to ask a question about battery heat dissipation. I am an HVAC engineer and I revied the heat dissipation of battery's in a battery room and noticed that our contracted had mentiond two figures. As an example I will take on of the battery room,s the first was 0,08 kW and the second was 1,786 kW this is the heat load for the same battery,s but under different conditions. Now I have questioned the contractor hhow tis is possible that there is a factor 22 between the loads and I recieved the following answer.
During float charge (normal operation) battery voltage is 2,25 V/cell. During boost charge voltage is raised to 2,4V/cell which means higher battery current than during float charge. Battery has it's internal resistance. As the internal resistance gives power looses equal to P= I^2*R this means that if current increases than power looses also increase ( battery resistance is constant). So boost charge requires higher voltage what causes higher current flowing through what produces higher power looses.

This was the answer I recieved from our contractor. The questions I now got is: is a differance in 0,15 Volt per cell sufficent to produce so much additional heat? And does the answer I recieved make sence?

Thanks for your help

 

[itsmoked]

That is correct. The batteries on float dissipate very little, or they would boil out into dry husks. But during charging they convert to heat about 40 percent of the energy put to them.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[IRstuff]

Yes, and Yes.

The only surprise is why there's only a factor of 4.7 between charge and float current. Your assumption about constant R might not necessarily be valid.

TTFN

FAQ731-376

 

[treez]

I assume these are lead acid batteries ?

....itsmoked...is it really 40% of the energy put into charging them goes off as heat?

.....i think possibly this percentage depends on the actual bulk charging current.

Max bulk charging current is usually C/4.....where C is the Ah rating of the battery.

....but i believe cutting the charging current is better for the battery.

..hollandhvac,......do you have switched mode chargers ?..if not then this may be upping the ambient.

..........float charging may be a waste of time if youre going to use the batteries pretty soon (couple of weeks) after the Over-charge taper current threshold has been reached. -you could just switch the charger out with a relay.....and then if charger is turned off (say overnight) the battery is not discharging through the leakage reistances of the semiconductors etc.

 

[itsmoked]

treez; That is the number as I recall. It is a sad number too. Always a bit depressing when you want to go off-the-grid solar and find you need to increase your panel wattage by about 50 percent. If you charge at some pedantic rate like C/10 I believe you can reduce this a little. You will notice that those heavy lead batteries warm up dramatically during bulk charging.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[jimkirk]

Sealed Lead Acid batteries and Lithium batteries can be charged with around 95% efficiency. Flooded Lead Acid are typically around 70% efficient. Slower rates yield higher efficiency and longer battery life.

 

[itsmoked]

I'm talking only of wet LA batteries.

The 70% you are stating jimkirk is the coulomb efficiency which avoids the further losses of the charger with its typically, less than, 90% efficiency, nor the regular equalization charge cycles that are very inefficient.

hollandhvac needs the total losses I would expect.

Sealed LA have higher charge efficiency but that is because they charge at a much lower rate, often too low for commercial requirements.



Keith Cress
kcress -

http://www.flaminsystems.com

 

[hollandhvac]

Additional info: the batteries are Sealed lead GEL and have 24 cells, the charge current is 70A

Question to IRstuff, what factor do you mean with 4,7?

Thanks so far for the answers.

 

[Noway2]

Based on the description given: 24 cell batteries, in a battery room, where AC is being applied, this sounds like the battery bank for a big UPS system. In these applications, the AC is used to keep the batteries at a temperature where they will have maximum output capacity. In this environment, the batteries will get a lot of special consideration with regards to charging, floating, and conditioning requirements. You should be able to get some accurate information regarding the power dissipation from the system vendor.

 

[IRstuff]

22 times the power is 4.7 times the current, assuming a constant resistance

TTFN

FAQ731-376

 

[hollandhvac]

And yes ITsmoked I am looking for the total heat loss of the batteries and why there is so much differance between float and boost charge.

 

[IRstuff]

That question was already answered in the 2nd response to your OP.

TTFN

FAQ731-376

 

[treez]

yes,

to elaborate, float charging is just "keeping it topped up"

it is the final phase of charging and sometimes omitted.

battery charging 12v lead acid has four phases:-

1. Trickle charge: if below 10.5 V (Constant very low current)

2. Bulk Charge: when reaches 10.5V (Constant high current, max C/4 (?) )

3. Overcharge: When gets to about 14.6V...(Voltage constant at 14.6V , current monitored while it falls)

4. Float charging:- When current falls to taper current threshold (some low current). Voltage is kept constant at the float voltage say 13.8 Volts.

Stage 2 is the high current stage....but it doesnt have to be if you dont mind slow charging

 

[TVtech]

If you are really not attached to the battery you can have some fun with it. I had a motorcycle battery that was low on charge but I had sold the bike a few months ago. I decided to see what would happen if I charged it at high amps. It was clear sided and I set it to 6 amps charge and watched as the plates curled and the coating stripped off them.

Of course I was young and foolsih at the time.

 

[raithrovers1]

If these are sealed gel batteries I would be surprised if they have a dual rate charging system. These type of batteries are designed to be charged at a single rate of approximately 2.27V per cell at a temperature of 20 degrees centigrade. The battery manufacturer does give details about higher rates of charge for these cells but they are very rarely used. You need to find out if the charger supplying these batteries does actually have a boost charge function. In my experience, I very rarely see sealed lead acid batteries that are supplied by a charger with a boost function.
Your ideal solution would be to see if the charger has a battery current limit function and if it does, what this limit is set to. Then contact the battery manufacturer and ask what the heat dissapation of the battery is at that charging current or find out the internal resistance of the battery (available on most battery data sheets) and work out the power dissapation from the equation that you have already stated.

Ups enginner for:

http://www.powerups.co.uk