Electric oil filled radiator

[DHutch]

According to Daniels figures, my batteries would last less than an hour ...

Yeah, which i think is actually quite an impressive figure!

- I dont think you would want to do it though, and a would actaully be very supprised if it would work.

- I dont have a very indepth knowlage of lead acids, but i would have though you would run into quite major problems with internale resistance?

- Most inverters cut out at around 10v, and even if you managed to obain negligable voltage drop to the batterys, i dont think it would be long till it got there, and it almost certainly would do the batterys that much good.

 

The batterys in our RC cars only lasted about 20 race meets or so before they start becoming noticably knackered.

- And many people only used each battery for one 5min race per meeting, possably two if your pushing it.

- But the high discharge rates, plus hard cycaling before the race meet, ment that after only a very short time, they where totaly gone!

 

For anyone thats at all interested, we're talking about discharging 3Ah sub-C batterys at 30amps down to 0.9volts per cell, and charging them at charge rates of 6-8amps or more. - Surfice to say, they did get a bit warm!

 

 

Daniel

[John Orentas]

Forget the calculations, there are some things that a plain wrong. Are we to understand that a bank of batteries are charged by some means, they then power an inverter which in turn drives a 2.5 kW 230 volt room heater.

 

This is madness, the capital cost alone should rule out this system before you even start looking at the massive losses.

Edited January 8, 2007 by John Orentas

[chris w]

Yeah, which i think is actually quite an impressive figure!

- I dont think you would want to do it though, and a would actaully be very supprised if it would work.

- I dont have a very indepth knowlage of lead acids, but i would have though you would run into quite major problems with internale resistance?

Daniel

 

It would work and there's no problem with internal resistance. BUT, it would only work for 27 mins (based on Blackrose's AmpereHourage) until the batteries would be at half charge and should then be recharged. (I corrected for the Peukert factor in getting to 27 minutes - it's not just AmpereHours divided by current).

 

Chris

Edited January 8, 2007 by chris w

[DHutch]

Forget the calculations, there are some things that a plain wrong. Are we to understand that a bank of batteries are charged by some means, they then power an inverter which in turn drives a 2.5 kW 230 volt room heater.

Oh year, deffonatly.

- As blackrose haas said, he doesnt plan to. However, purely from a point of curiosity, its interesting to discuse, possably even experment would bight happen.

 

 

 

Daniel

[Theo]

It would work and there's no problem with internal resistance. BUT, it would only work for 27 mins (based on Blackrose's AmpereHourage) until the batteries would be at half charge and should then be recharged. (I corrected for the Peukert factor in getting to 27 minutes - it's not just AmpereHours divided by current).

 

Chris

 

 

Could you tell us a little about the Peukert factor?

 

Nick

[bottle]

From Wikipedia

 

 

 

Peukert's Law, presented by the German scientist W. Peukert in 1897, expresses the capacity of a battery in terms of the rate at which it is discharged. As the rate increases, the battery's capacity decreases, although its actual capacity tends to remain fairly constant.

 

Peukert's law is as follows:

 

 

where:

 

• is the capacity according to Peukert, expressed in Ah.
  
• is the discharge current, expressed in A.
  
• is the Peukert constant, dimensionless.
  
• is the time of discharge, expressed in h.
  

For an ideal battery, the constant k would equal one, in this case the actual capacity would be independent of the current. For a lead-acid battery, the value of k is typically between 1.1 and 1.2 however.

 

The Peukert law becomes a key issue in a battery electric vehicle where batteries rated at 20 hour discharges are used at much greater rates of about 1 hour.

 

Watt-hour counters can be programmed with battery capacity, Peukerts exponent, and other particular batteries characteristics. They then count watt-hours and can display various information relating to the state of charge of the battery pack, including remaining energy and thus range.

 

But I am sure Chris can explain it better.

[chris w]

Peukert Factor, in simple terms: (I hope)

 

The label on a battery will state something like capacity = 100AH (ampere-hours) @ the 20 hour rate.

 

What does this mean? What most people take it to mean is that one can draw 5 amps for 20 hours (= 100AH) and therefore one could draw 20 amps for 5 hours (=100AH) or say 1 amp for 100 hours (=100AH) and so on.

 

Unfortunately this is not true. It's not true because a battery's discharge characteristic would have to be a straight line for the above to hold. ie: the same amount of charge is taken for a constant current at any point in its discharge cycle.

 

Well, unfortunately, a battery's discharge shape is a curve, a really steep curve that doesn't even approximate to a straight line, it's an exponential curve to those for whom that has any meaning. The manufacturers therefore simply pick one point on the curve and state its value. By convention, this is usually the 20 hour rate (although some marketing blurb will use other figures to make the battery look bigger than it is). At the 20 hour rate point (in this case 100/20 = 5 amps) the figures are correct, ie: drawing 5 amps for 20 hours will completely discharge the battery.

 

However, if one drew say 20 amps, (expecting 5 hours to complete drain) one would be very disappointed to find that the battery would be completely exhausted in only just over 3 hours. And conversely, if one drew a current of only 1 amp expecting the battery to drain out in 100 hours, one would be very pleased to find it actually lasted for over 160 hours.

 

Please note, by the way, that you should never allow your batteries to fall lower than 50% charge to avoid damage to them so the times above should be halved in reality for practical use.

 

The actual capacity of a typical "so-called" 135AH battery (a typical boat version) is around 240AH. The 135AH is simply derived from the fact that the manufacturer found (designed) that a current of about 7 amps exhausted the battery in 20 hours.

 

If the current drawn is close to what I shall call the "Peukert" current (in this case 7 amps) then the battery will last as one would expect even ignoring Peukert ie: 135AH. For currents larger than the Peukert current the battery will last less than simple arithmetic would predict. For very large currents, much less. Conversely, for currents much smaller than the Peukert current the battery will last far longer than simple arithmetic would predict.

 

If you look at Gibbo's webpage here he has a good in-depth article on the subject.

 

Note further, that large battery banks are far better than several smaller ones as the current drawn is then a smaller percentage for each battery and therefore closer to or below the Peukert current and hence one will get longer discharge times out of the battery bank.

 

Beware of batteries that quote an AH figure at other than the 20 hour rate. For example a battery labelled as a 100AH @ the 20 hour rate could also be called a 145AH battery at the 100 hour rate. Both would be true, the manufacturers have just picked a different point on the curve. However, always compare like with like in hour rates otherwise the comparison is meaningless (other than to a mathematician). As I said above, the vast majority of batteries are quoted at the 20 hour rate.

 

Chris

Edited January 8, 2007 by chris w

[bottle]

See I told you he would/could

 

 

 

 

Thanks Chris

 

It all made sense to me.