Did You Know? Switch Mode Rectifiers, Calculating Float Voltage & C-Rates
Switch Mode Rectifiers
The most common technology in use today for stationary battery charging is the Switch Mode Rectifier (SMR). They offer a higher efficiency and are smaller and lighter than the legacy Ferro Resonant and Silicone Controlled Rectifier (SCR) types. More importantly, they are easily field-swappable. Did you know that the SMR was first invented 110 years ago and was a mechanically switched version used in the Cadillac motor car? Did you know that the Cadillac was named after a French explorer named Cadillac who is said to be the founder of Detroit, MI. The famous colorful Cadillac crest is based upon his coat-of-arms.
Calculating Float Voltage
Talking about battery chargers, did you know that the Open Circuit Voltage (OCV) and consequently float voltage of a stationary lead-acid battery is dependent upon the Specific Gravity (SG) of the battery electrolyte? The same rule applies to both VLA and VRLA types. If the SG is known, simply add 0.845 to the SG. For example, say the SG of a battery cell is 1.300. Add 0.845 to this and you get 2.145, which is the OCV of the fully charged battery cell. In order to compensate for losses when the battery is in service, a voltage higher than the OCV has to be applied to the battery. This is known as the float voltage. The added voltage is typically 120 mV, therefore, in the above example of the battery cell with an OC voltage of 2.145 V, then the float charge voltage would be 2.145 plus 0.120 which would indicate a required float voltage of 2.265 V.
What Does C-Rate Mean?
I often get asked what the “C-rate” means with respect to batteries. Well, as with many battery issues, it can be confusing. Often the “C” is confused with Coulombs, which is the battery’s capacity to store a charge. But with respect to C-rate, it is a measure of the rate that the battery is charged or discharged in Amps stated in multiples of the battery capacity. For example, say a battery has a capacity of 100 Ampere hours at the 8-hour rate, then a C/10 discharge (or recharge) current would be 100/10 or 10 amps. So, if a data sheet for a particular battery states that the recharge current should not exceed C/5, that means that for the 100 Ah example, the recharge current should not exceed 100/5 or 20 amps.
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