Website for Turnkey Solar Solutions in Arizona

Frequently asked questions (FAQ).

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-charging. Very high or low charging rates will affect the ideal settings for a battery due to internal resistance and reaction rates outside the anticipated range. ETA designs systems for a nominal five days of sun-less autonomy and a charging current of about seven Amps per 220 Amp-Hours of battery capacity. An undersized battery requires a higher voltage setting since the internal IR voltage drop will be higher than normal. Call us if assistance is needed for the proper settings. One year of extra battery life will more than pay for the added cost of the charge regulator features or custom settings!

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-off" rate is fairly rapid, however it will be much longer when at 100% capacity. ETA uses this property to good advantage. We let the battery tell the regulator when it is ready to accept additional charge. As a result, when the battery is approaching full capacity we are still charging most of the time. However, at full capacity, we may be charging less than 1% of the time. If there is a small load on the system, the regulator automatically adjusts and increases the charging duty cycle to keep the battery at full charge. In this manner, the Pulse Width Modulation (PWM) rate automatically adjusts to the battery requirements.

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-discharge rate. Other designs use high frequency charging methods and short pulse widths that generate more Electromagnetic Interference (EMI) and do not allow time to measure the surface charge bleed-off rate.

Battery longevity not only depends on design and specific chemistry, but on the usage of the battery. Lead-Antimony alloy positive plates give the most cost-effective design for deep-cycling applications. For most applications five days of autonomy results in a satisfactory compromise. This infers a daily Depth-Of-Cycle (DOC) of less than 20% since the battery will normally be charging for a third of the day. Battery life decreases significantly as the daily DOC increases. For this reason the industry normally recommends a maximum depth of discharge of 80%. Some of our controls have a factory settable Low Voltage Disconnect (LVD) that removes power from the load when the battery reaches 11.7 VDC, for a lightly loaded 12 VDC battery. Custom settings are available. We have found that system reliability is improved when field adjustment is not an option.

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-out mechanisms, and welded contact problems of relay type designs. Up to four solid state transient absorbing avalanche diodes (1.5 Joule ea.) protect the regulators from lightning induced surges and inductive load voltage spikes. Our pulse width modulation, switching shunt design is easier to protect from lightning surges than a design which uses a series-pass switch. All regulators incorporate a conservatively rated blocking diode to prevent discharge back to the PV module(s) at night. This meets the NEC requirements for safety if the module wiring becomes shorted.

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--- normal operation with no battery attached. This design feature is included for two reasons, first the regulator will not overheat at this low switching rate, and any loads (appliances) will be protected from open circuit PV voltage which could cause failure. Note that series-pass regulators normally do not have this feature. ETA designs products that are not only cost-effective, but will exhibit the best system reliability obtainable.