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ETA Engineering, Inc.
4049 E. Presidio St., Suite 117
Mesa, AZ 85215
Phone: 480-
Toll Free: 1-
Fax: 1-
Note: Prices are subject to change without notice due to changes in other vendors' prices to us. These changes may arise from normal product development plus fluctuations in foreign exchange rates and commodity prices.
ETA Engineering Battery Charging Methodology
by Lane Garrett, P.E., C.E.M.
Our experience with various types of batteries dates back to the 1950's. ETA maintains an extensive library of manufacturers' data and test results covering most battery chemistries. Our studies and field experience have helped to optimize battery charging and control for maximum life, using alternative energy charging sources, especially Photovoltaics (PV).
There are numerous factors to consider in the design of a charging system when minimizing total costs over the battery lifetime:
Temperature affects the lifetime of a battery, significantly shorter at high temperatures
and longer at cold temperatures. Temperature also affects the internal chemical reaction
rates and therefore the internal resistance and efficiency for higher rates of charge
(or discharge). A battery is significantly less efficient under heavy discharge at
cold temperatures. The necessary voltage to fully charge a battery drops significantly
at higher temperatures. In the heat of summer, maximum recommended charging voltage
can drop to under 14 VDC for a 12 VDC deep cycle battery. In the cold of winter,
it can take 15 VDC or more to obtain full charge. For this reason, all our regulators
are temperature compensated to automatically adjust the maximum allowed charging
voltage. The voltage vs. temperature curve is adjusted for the type of battery chemistry
at the factory to avoid field adjustment. This factory adjustment also optimizes
the tradeoffs between water usage, charging the battery fully in minimal time, and
equalizing individual cells to full capacity. Our settings are high enough to cause
the electrolyte to bubble, preventing "stratification" which can ruin a battery in
one winter of under-
When a battery approaches full charge, additional "surface charge" builds up on the
plates which reduces the rate of charge acceptance. When charging with a constant
current source such as PV, the battery voltage will increase significantly and the
bubbling will become more rapid. If the charging is halted, the surface charge will
"bleed" off at an inverse rate to the percent of battery charge. If the battery is
at 96% of capacity, the "bleed-
It should be noted that the regulator only applies full charging current and does
not switch to a trickle charge mode. This is done for an important reason. Trickle
charging does not cause uniform current density across the plate surfaces and tends
to increase current density at the top of the plates or anywhere there is less resistance
between the plates e.g. where faults in the plate separators occur. In the past this
contributed to the formation of dendritic growths between the plates causing internal
current flow and unbalancing of the battery cells. This is not as much a concern
with today's improved separators, however higher charge rates are desired to maintain
even current density across the full surface of the plates, maximizing battery life.
This method of low frequency PWM is battery controlled and therefore adjusts as the
battery ages and increases its self-
Battery longevity not only depends on design and specific chemistry, but on the usage
of the battery. Lead-
Extensive Quality Control, Conservative Engineering with large safety factors, give
the ETA family of regulators unsurpassed reliability that has been proven by field
experience dating back to 1978. All regulators will withstand twice their current
rating for a third of a minute without harm. (This is equivalent to four times the
rated power dissipation). ETA regulators are built using a patented design. All solid
state circuitry prevents the higher current draw, wear-
If one of our regulators is disconnected from the battery (during daylight) the output
voltage will quickly rise to the maximum setting allowed and "shunt" the PV source
to prevent the output voltage from rising any further. A significant delay is built
into the circuitry before the regulator will attempt charging again. At that time
the output voltage will again come up to the maximum setting. The regulator will
oscillate between charge and shunt at a period of a few Hertz and have a low average
output voltage that may read around 5 VDC for a 12 VDC regulator-
©2001-
Last updated: December 19, 2008