Battery Power Systems Design

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Battery-powered systems are necessarily complex for three reasons: charging, discharging, and heat management. In the world of lithium batteries, this is primarily managed by the BMS, or battery management system.

Panasonic gives excellent battery management advice for dealing with NiCd and NiMH types and the advice is applicable to SLA, VRRH, and other lead-acid battery configurations as well.

Battery heater:

This is a battery heater, which is a necessity in Alaska. The catch with battery management is always keeping them cool when it is warm out, and keeping them warm when it is cool out. The fact that they are primarily lead and have a large amount of thermal mass does not always help. Obviously, passive methods of cooling are good but have to be custom-designed for the application. In the case of NiMH batteries, high temperature at the end of charging is a normal by-product of charging, but should be managed to prevent damage to the cells.

Charging should be structured to state of charge, particularly making sure that each individual battery is charged to it's proper voltage. Strings of batteries mitigate the effects of a shorted cell by minimizing the resulting overcharge voltage. However, a battery going into an open-cell or high-resistance state causes all batteries to be effectively disconnected from the charger or load, rendering a larger capacity of batteries unusable. I recommend that a three-bus approach be used, where battery packs are attached to a load bus using ZVS or other style of buck-boost converter or switch, a charger connected to that load bus, and the battery under charge isolated from the load bus and connected to the charging bus when under charge. This permits a rotating "once a week" charge of each pack, even if the power is out, with a terminal behavior of removing packs from the discharge set as packs are charged for the last time. This way, as few cells as possible are "killed" by being left in an uncharged state unless the load dictates such. Entropy: you can't avoid it.

Discharge should be figured to the chemistry of battery as well. In the case of NiCd and NiMH chemistries, a terminal voltage of 1.0 or 1.1 V per cell may be the lowest that should be permitted. In the case of 12V VRRH or SLA batteries, a terminal voltage of 10.5 or 10.8 volts should be used depending on the current being drawn and the resistance of the wires between the batteries and the load. Once a battery is discharged, it should be recharged as soon as feasible or possible to prevent effective battery death.

Temperature management has two extremes: cold and heat. Even those are relative to the environment of the battery depending on the location of the system in the world. Death Valley regularly has extreme temperatures exceeding 120 degrees F, and the American South may see 90-100 degree F temperatures during a typical summer. Likewise, low temperature extremes are more likely in certain areas than others. These factors shape the design and implementation of a battery thermal management system.

Heat pipes and solid/liquid metal (gallium, indium) cooling systems may be used as pack temperatures approach 90 to 100 degrees F. However, these systems have to be designed not to operate if temperatures are cooler than 70 degrees F as battery capacity starts dropping off below 65 degrees F. Paradoxically, many battery chemistries tend to generate heat as they are discharged which is a benefit to low external temperatures but a caveat when external temperatures are warmer. However, heat-pipe systems may be made of any number of materials, and have correspondingly different behaviors depending on the substances involved. For instance, propane boils at -43.6 degrees F (-42 degrees C), which means it can act as a vapor-phase cooling system if one end of the system is compressed to a liquid state. Likewise, the temperature of the system can be held to a constant as long as a liquid is boiling into a gas state, such as a pot on a stove full of water. This is how a double-boiler manages effective temperature control. In the case of using water, the temperature of the water must not be allowed to drop below 33 degrees F or ice crystals can form at extremes.

Additionally, geothermal contact may be involved in temperature management, using loops of electronegatively inactive, grounded, non-oxidizing metals in well casing, or direct contact with rock outcrops or burial below the soil surface. Depending on the materials involved, these systems can be efficient and/or maintenance free or require periodic maintenance to renew materials or sacrificial anodes. 

In the case of an outside temperature below 55-degrees F, the outside air temperature is lower than the temperature of the ground, which makes contact with ground a good thing. In the case of an outside temperature above 55-degrees, contact with the ground is not desirable because battery capacity decreases rapidly below 65-degrees F.

The other layer of complexity to battery management is that of corrosion. Chemicals in batteries can corrode various metals to different degrees, and it is desirable to prevent some metals from reacting with some electrolytes. Further, some batteries require venting, which may be implemented simply using fans or by using aquarium tubing in others. In general, one should assume that batteries will eventually fail, and in the case of SLA, VRRH, and lead-acid batteries, that either hydrogen and oxygen gas will be vented, or sulfuric acid will be vented or drain into the enclosure. Any nearby electronics will be damaged by the gasses from the batteries.

Yet another area of concern would be the degree to which small rodents of the area find the battery box to be habitable in any and all states of weather.

Another complexity is that batteries should not be placed in parallel unfused lest a battery develop a shorted cell, or several cell failures following a shorted cell. When a 90 Ah battery is rated for 2,000 amps into a dead short, one can be certain that a battery failure will result in the venting of charging gasses and electrolyte fumes while the good battery forces power into the dying battery.

Additionally, serviceable battery types should be serviced on a periodic basis, with distilled water added as appropriate and electrolyte when necessary. Serviceable batteries should be vented and humidity levels managed. Non-serviceable batteries should have discharge and charge rates limited to prevent cell over-temperature or over-pressure.

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This page contains a single entry by Kris Kirby published on November 5, 2016 5:23 PM.

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