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Tech Info - Rechargeable Batteries

Two 75 AmpH 12V Lead Acid BatteriesThe batteries are the weak points for many EV's – the general problem faced by the vehicle designer is the considerable volume or mass of battery needed to store the amount of energy the vehicle will need to keep it going between charges. Some battery technologies are better at this than others i.e. some have higher “energy densities” than others, and, yes you guessed it, the cheaper battery types are generally the least capable. Rechargeable lead acid batteries are the norm for most DIY builds – they are easy to get, easy to charge and are the least expensive to buy - as are their chargers. They also come in a fair range of sizes (capacities) which allows some flexibility in their packaging within the vehicle. Other battery types that might be considered are, for example, Ni-Cad, Ni-MH, Li-ion (and others) although you will need to do your homework before constructing powerful enough packs and make sure you use compatible chargers. You can also expect to dig deeper into your pockets. If you are interested in these higher tech battery types check out the robot builders, they are used by some to save weight in their robot designs.



If going down the lead-acid route you will find a large range available from shops supplying mobility scooters, golf carts etc and plenty on-line. Be sure however to use “cyclic” duty or “deep draw” types. These are specifically made for vehicle applications and are designed to be discharged deeply and then re-charged over several hundred cycles. Their internal construction is a bit different from standby or float type batteries which generally do no see this cyclic duty. Conventional automobile starter batteries are not cyclic duty batteries.


A pack of 4 38Amph 12V batteriesYou'll find that some makes are more expensive than others which may reflect the increased number of charge/discharge cycles the battery can sustain before it deteriorates and looses its ability to hold charge. It is advisable to obtain the full spec sheets for the batteries concerned to compare life, discharge currents, charge currents etc. Mostly I use 12V batteries connected to give 24 or 36Volt supplies. 17AmpH and 38AmphH are our most used sizes but there are bigger units readily available up to about 75AmpH (in our 1300W yard tractor) and heavier for larger vehicles.

Proper charging arrangements are important for prolonging battery life. Get a multimode charger specifically designed for deep cycle lead acid batteries. These will have typically three modes of charge; a constant current main charge, a constant voltage finishing charge and a float top-up charge (at a lower constant voltage) to keep the battery topped-up as long as it is connected to the charger. These will not overcharge the battery and can be left connected for long periods of time. 12V and 24V units are available with 12V being more common. Even though you have a 24V pack you can still use a 12V charger although it make require some connections to be broken in the pack. For a good description of charging options for battery packs check out Battery Tender's article on connecting and charging battery packs here.

Safety Note: Although the voltage in these battery packs is relatively low the current that can flow in short-circuit conditions can be enormous. The unfortunate conductor (metal tool, misplaced cable etc etc) that is causing the short circuit will get extremely hot with potentially dangerous consequences. Be very careful not to cause a short circuit across the +ve & -ve terminals of either individual batteries or the full pack.


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