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[blackrose]

10 hours ago, WotEver said:

What is it...

The charging current going into the Batteries towards the end - the tail - of the charge. 

How to assess it / what should it be...

You should be looking for the current to reduce to around 1-2% of the nominal battery capacity. This is simpler than it sounds. For a 120Ah battery you wait for the current to drop to 1.2A to 2.4A. For a 500Ah battery, 5-10A etc. It may be that the tail current never drops this low with old batteries, in which case you look for it to have remained at a low stable value for around 45 minutes. It is important that you must measure this while the charge source is outputting its normal charging voltage of 14.4V or above (known as the absorption voltage). The charge source must not drop to float voltage (13.6V or thereabouts) until the tail current has dropped to this level. Many (most?) chargers drop to float way too early. 

Where is it measured...

On one of the leads to the battery bank. Either negative or positive, it doesn’t matter, you’re measuring the current flowing into the battery. 

How is it measured...

With an ammeter. This can be in the form of a handheld multimeter although that’s not very convenient, or a standalone shunt based ammeter that you have installed. Battery Ah counting gauges such as the BMV all incorporate a shunt based ammeter so they could be used too. 

A Smartgauge specifically will NOT measure tail current as it does not measure current at all; it indicates the charge state of your batteries when you are using (discharging) them by measuring voltage. 

Have a read of this:

 

I'm still not sure why one would want to measure tail current? If it does (or does not) reduce to 1-2% of nominal battery capacity, what does that actually indicate? How is it useful?

Edited February 19, 2018 by blackrose

[WotEver]

2 hours ago, blackrose said:

I'm still not sure why one would want to measure tail current? If it does (or does not) reduce to 1-2% of nominal battery capacity, what does that actually indicate? How is it useful?

It tells you when your batteries are nearasdammit full and when you can therefore stop charging them. There are three methods to accurately determine 100% charge:

• Measure the specific gravity (relative density) of the electrolyte in every cell. 
• Measure the off-load battery voltage when the surface charge has completely dissipated. 
• Measure the tail current. 

The SG measurement is inconvenient, time consuming, messy, and potentially dangerous. The voltage measurement is time consuming and once measured, if they’re not fully charged, you have to start again. The tail current measurement is instantaneous and simple.