Standby batteries are subject to a float current all their service, lives in order to maintain them for instant use. The resulting float voltage is actually an overvoltage of approximately 150 millivolts per cell on top of the cell’s fully-charged natural open-circuit voltage. Cell internal resistance/impedance is calculated by drawing a current from the cell in a pulse pattern and measuring the change in the terminal voltage. If too low a test current is used, several factors which can adversely affect the measurement come into play The change in terminal voltage will be too small to come from the cell itself, but will come from the overvoltage, mainly influenced by the charger. If the float current changes the measurement will be different. Battery system noise and ripple can obscure a small test current and lead to adulterated readings A low test current means a small response signal; the smaller the signal the less accurate the measurement can be.