Capacity Testing - Discharge testing batteries

You can not test deep cycle batteries with handheld test equipment in a matter of minutes. It takes 8 hours to ensure your batteries are fully charged before spending 8 hours or more discharging them before another 8 hours to recharge. (assuming the batteries are OK), if they cannot hold capacity these tests can be done in hours once you have ensured the battery is first fully charged because the discharge and recharge time will take a much shorter time due to no capacity.

How do you perform a test, there are many variations, some include:

1. Reserve capacity test. Reserve capacity is a discharge at 20deg C and 25A constant for X number of minutes which will be stated on your battery as RC. If RC is 200 then that's 200 minutes at 25A discharge until the battery reaches an end voltage of 10.75v. If your test lasts 100 minutes then your capacity is 50% of its rated new Ah.

2. AH rating test. If a battery is rated at 200Ah @ 20 hours that's 5 amps per hour for 20 hours till an end voltage specified by the battery manufacturer. They often quote in VPC or volts per cell. So for a 12v battery you need to multiply the number by 6. 1.8vpc = 10.8v when testing a 12v battery. You need to discharge a fully charged battery at 5 amps for 20 hours and if the battery lasts 10 hours before the voltage reaches 10.8v then that's 50% of its rated capacity.

3. Not recommended but some distributors supply 75A discharge testers because that's a test used on golf cart batteries that they distribute. Golf cart batteries are also normally 6v 250Ah or larger (can be 415Ah) so a 75A discharge test is only 1/2 or less the batteries rated capacity. If you perform a 75A constant current discharge test on a 100Ah battery you certainly aren't going to get a real-world test result on a 100Ah battery when the test would only last 30 mins on a brand new healthy battery but your tested battery only lasts say 17 mins. It's easy to quote a large percentage number differential or failure when you've only tested for a few minutes and each minute represents 2-3%. So this test needs to be put in perspective of reliability to your situation. It is, however, a great way to ensure you buy a new battery from the battery test company.

Last point, a discharge test is using a deep cycle, your battery only has X number of those depending on how you've used your battery so they really only need to be done when there's an issue or if performing a system upgrade and you want to assess the suitability of the batteries to perform their new duties.

Ampere Ratings or Amp Hour Rates

Batteries are classed, categorised and rated to international standards. Those standards can also be manipulated when published if you state your variances, the trick is to know what's important and why.

In the Aviation industry battery is rated at the 1 hour rate because if your batteries fail while in flight you better hope you can get on the ground within 30 minutes before you lose communication or navigation instrumentation. This means a battery rated a 10Ah @ the 1-hour rate can provide 10A until it's voltage reaches 10.02 volts or 1.67 volt per cell and is considered completely flat. (sometimes the end voltages are much lower to make a batteries capacity seem better).

Traction batteries like used in Electric Forklifts are rated at a 5 or 8-hour rate because depending on the type of products being picked the equipment might average 5 used hours in one shift. Other products might require more handling in an 8-hour shift so you would use the 8 hour rate. Either way the end result would be flat batteries by the end of shift, which once recharged is 1 cycle. If you use larger batteries you can lift the depth of discharge from say 50% to 60% and that will manipulate the depth of discharge (end voltage when flat) and the increased cycle life from say 2000 cycles to 2500 cycles will extend the replacement time (in years) and your Return On Investment (R.O.I).

Industrial Applications use power over a working day with lunch breaks so a 10 hour rate is used. i.e. 18Ah battery at the 10 hour rate will provide 1.8A of power for 10 hours until the voltage is reduced to 10.02 volts. (If the temperature is higher or lower it will change the batteries rated capacity).

General use, deep cycle & Automotive/Marine batteries use the 20 hour rate or Amps over 20 hours so again you divide the stated Ah by 20 hours to get the average current discharge per hour. 100Ah / 20 hours = 5 Amps per hour. (All to often we see all the ampere ratings, temps, end voltages, and battery purposes cross over to confuse and or mislead a buyer as to how good a battery will perform not to mention the variations in a batteries weight).

Solar use the 24 hour rate for daily averages, but the 100 hour rate or 120 hour rate are more common as they refer to a better average use over 4 or 5 days.

You can see how when batteries are advertised at a rate that differs from these industry standards that the designed purpose of the battery may have been for a different use and that it may mean the charging rates need to be different, the voltages might need to be different or that the cycle life could be more or less than your expected use with consequences good or bad.

DC Isolators in Solar Arrays and Circuit Breakers

The Electrical standards for ultra-low voltage state that both the positive and negative conductors (wires) of a solar array need to have a disconnection function. (this means the ability to be turned off) When the panels are producing energy disconnecting under "load" will cause an arc as the contacts are opened. Automatically reconnecting circuit breakers can become welded in place while connecting or disconnecting under load which is why DC Isolating Switches are used, being manually operated you will know it's still working correctly as you turn the rotary on and off. However, these DC Isolators are expensive, more expensive than using a manually operated circuit breaker which you can manually trip and reconnect. This is why we supply double pole (pos and neg) circuit breakers in housing as its a 2 birds with one stone approach. Protection and Isolation.

Solar panels themselves can't short circuit even if you connect the two outputs of the panel together as the internal wiring is rated higher than the panels output. If, however, you have 3 panels in series and one fails. The power output of the other 2 panels will be more current than what a standard 16A circuit breaker would be capable of handling. For this reason, circuit protection may not be required, but isolation is. On 1 or 2 panels a single 16A 500VAC circuit breaker is adequate. But with 3 panels, you should isolate each panel with a circuit breaker to ensure safety and isolation can be achieved.

At this point, 3 circuit breakers cost more than 3 fuses and 1 x DC Isolator so the choice is yours to make. Isolate and protect yourself.

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