PHP Warning: Use of undefined constant VBA_SCRIPT - assumed 'VBA_SCRIPT' (this will throw an Error in a future version of PHP) in ..../includes/functions_navigation.php(802) : eval()'d code on line 1 Wire size to charging battery? - Page 5
SOrry oldboot, just out of curiosity can you point be to the manufacturers that quote 4 hour charge times please. Most of the material I have seen points to about C/3 and about 12 hours. I would like to factor this into my charger design.
BTW. I have dropped the LT1513 from my design, they are about $20 a peice one off, so going to use a PIC or similar microcontroller at about half that cost.
I have a fair bit of experience with fast charging SLA batteries, so let me try and explain why fast charging SLA is a struggle.
When you put a decent amount of current into the battery the cell voltage will quite quickly get over the safe cycle charge voltage. So when this happens you really need to switch into constant voltage mode to get the rest of the charge into the battery otherwise the SLA battery will start to make a hissing sound which is gasses venting from the battery..
With 0.3C charge rates you will find the cell voltage will quickly get above the safe cycle cell voltage,
The chargers I have made manage to almost fully charge a SLA battery within 16hours. (I charge a 6V 4.5AH battery using a microprocessor - starting current is 0.1C - I limit this due to the limited smps output power i have on tap). It would be possible to go faster, but not too much as eventually the cell will hit the point where Constant voltage is the best way to charge (as the cell voltage has reached the safe cycle voltage limit). Once you hit constant voltage the battery regulates its current intake and it drops relatively quickly getting the last of the charge into the battery takes some time- this is why fast charge times are difficult to achieve on SLA.
To help me explain, attached is some data from the one of the chargers I have made (I have added in the current line to explain what the current does during this type of charge also)(please also note you should terminate the cycle charge voltage after extended periods of charge, also the charge current limit on this graph was dictated by me and is by no means applicable to all batteries):
Why is it then that manufacturers state in black and white a 10 hour charge rate, then state for between 10 and 14 hours.
It may indeed be possible to charge a new battery at the 10 hour rate in less that 12 hours, but as the battery ages the charging efficeincy will be reduced.
In addition even at the 10 hour rate, as the battery approaches a state of increased charge the charge rate will naturaly reduce ( unless constant current charging) as the battery terminal voltage approaches the charging voltage.
This is 1st year electrical theory.
If you want it in black and white I refer you to panasonic application note "Panasonic VRLA charging methods".
Where it provides charging curves and states with regaard to constant voltage , current limited charging, which it refers to as "constant - voltage and constant current charging method" which closely approximates what we are talking about.
" This method charges the battery by controlling the current at 0.4CA and controlling the voltage at 2.45 V/per cell. Proper charging time is 6 to 12 hours depending on discharge rate"
Most of the manufacturers have similar data and application notes on their web sites, they are very easy to find.
For their data just google " panasonic SLA batteries" & wade in.
While 4CA is specified and permitted in specifications, personaly I would be happier to charge at a lower rate, while charging a 7Ah cell at 2 to 2.5 amps may be quite fine, I would be happy to charge at 1 to 1.5 amps. Most of the equipment I encounter would do this mainly because it is a convinient thing to do with available components.
I am in the business of dealing with facts, not myths or outdated preconceptions.
If I am incorrect, I am happy to be corrected, provided I am supplied with reasonable evidence.
I thank oddbudman and therealandy for their learned responses and his encouragment to check my facts, I have learned things important to me.
I have checked my facts by refering to manufacturers data, making enquiries and performing a couple of simple experiments.
I shall be continuing with my investigations into matters regarding SLA batteries as I feel the need to update and refresh my knoweledge.
In this day and age, things change, somtimes rapidly.
I also often find many perceptions and ideas old and new that have been preached as fact, were flawed or incorrect from day one.
Posting Permissions
Reply With Quote