Always disconnect all circuits before working on any power system.
Always follow the correct re connection sequence:
1 positive connection to battery
2 positive connection to load
3 negative connection to battery
4 negative connection to load
Typical 12 volt lead acid batteries have a voltage of about 14 volts when fully charged and 11 volts fully discharged. Most landmobile and public safety radio equipment doesn't operate properly below 11.5 volts. Oversized loads or excessive duty cycle cause rapid depletion of battery capacity. Battery systems must be sized to the load in order to supply the current needed.
Cold Cranking Amps (CCA) represent the current an automotive "starting" battery provides continuously for 30 seconds at 0 degs. F before voltage drops to 1.7 volts per cell (Vpc), at which the battery is fully discharged.
Marine Cranking Amps, are measured at 32 degs. F. Cranking amps tell nothing about how long a battery will run your equipment. Reserve capacity is the time a starting battery will sustain a continuous 25A load before cell voltage drops to 1.7vpc.
The performance measurements used for rating deep cycle batteries are amp hour capacity and depth of discharge (DoD). Amp hour capacity is total current available over time, measured at 80 degs. F. DoD is the percentage of capacity available during a charge discharge cycle.
Amp-hour ratings of deep cycle batteries are based upon a discharge rate at 1/20 of battery capacity. This is expressed as "C over 20". A marine battery rated 200ah at C20, discharged continuously at 10 amps, at 80 degs. F., will sustain that load for 20 hrs. Engine starting batteries are designed for 20% DoD, gel cells 25%, "deep cycle" batteries from 50% to 80% and telecom or aviation flooded NiCds 100%.
Starting batteries perform poorly for communications because they are designed for short periods of high load. Deep cycle batteries are much better. A 100w VHF repeater, drawing 20 amps on transmit, requires a minimum 100ah battery to stay within a C20 discharge rate, at 80o. F.
Lead-acid batteries lose 50% of their capacity at 32oF!
A BCI Group U1 (25 lb., 31ah) gel cell is well balanced to power an FM mode, Marine band VHF radio at 25% duty cycle, medium power transmit, requiring about 6A for 25w transmitter outpit, which approximates a C/20 rate of discharge. If Tx output is increased to 50w, as in a high band VHF LMR radio the current load increases to 10 amps, representing a mildly oversized load approximating C/10. This is OK for intermittent surge use, but stressing a battery frequently shortens its useful life. Deep-cycle, flooded lead-acid marine batteries tolerate C10 with some loss of life cycle.
A satisfactory rule of thumb for sizing communications battery systems for a C/20 discharge rate is to ensure one amp-hour per watt of transmitter output over a 12-hour operational period.
Lead-acid batteries at normal ambient temperature should be charged with a current from 1/10 to 1/20 of their capacity. When not in service, lead acid batteries self discharge at rate of about 5% per month. The rate of self discharge increases with the temperature. If a lead acid battery is left in a deeply discharged condition for a long time it becomes "sulfated" as sulphur in the acid combines with lead from the plates to form lead sulphate.
Sealed lead acid (SLA) batteries include gel cells and absorbed glass matt (AGM), have stabilized or "starved" electrolyte, are valve regulated and completely sealed. Because there is no free liquid electrolyte to spill, the battery can be used safely in any position. SLAs are much safer than flooded types for indoor use and in sensitive equipment such as computer uninterruptible power supplies, which would be damaged by exposure to acid fumes. Any sealed battery will vent if overcharged to the point of gassing, because valves are designed to purge pressure from inside the battery case.
Automotive constant current chargers intended for flooded batteries must not be used to charge gel cells unless they have voltage limiting circuitry to preclude their exceeding 14V during charging.
The use of properly sized wire and appropriate connections is important in DC power systems. DC polarity must be maintained throughout the system, as must color coding conventions of wire insulation: positive red, negative-black and equipment ground green or bare, following the wire color conventions used in automobiles.
Wire gages are much larger than in typical AC systems, because undersized wiring causes excessive voltage drops which result in loss of available power, which causes some loads to work poorly, or not at all. For instance, if too small a wire gage is used between a charge controller and battery, the voltage drop measured during full charging rates reduces the set point the battery is recharged to, reducing operating capacity and life cycle.
To minimize the effects of voltage drop, keep cable runs as short as possible. For instance, in a 12 volt system with a 10A load, such as typical landmobile radio transceiver, the AWG #14 wire used in typical vehiucle installs results in a 5% voltage drop over 11 ft. AWG #10 has a 2% drop over the same distance and 5% drop over 18 ft. If battery leads must be extended to reach a trunk-mounted transceiver, use wire 2 gages larger for as much of the distance as possible.
All splice connections must be secure and able to withstand vibration, moisture and corrosion. Splices of wire up to AWG#8 should be overlapped for a length not less than five times their diameter, spiral wrapped at least three turns, soldered, covered with shrink wrap or electrical tape and then waterproofed. Larger gage wires should be overlapped and connected with split bolts, soldered, covered with electrical tape and then waterproofed.
NEVER hammer cable connections onto terminal posts! This breaks fragile spot welds between terminal posts and plates, causing shorts, which could cause a spark and ignite free hydrogen gas, causing an explosion! Inspect all caps for sound good condition, replace and tighten securely by hand only. Tighten battery tie-downs securely, but not so tight as to distort the case.
Always determine why a fuse blew before replacing it. Proceed logically, check the most obvious things, such as r excessive voltage drop at the load. Knowing what failed is necessary to avoid repeating the condition causing the failure. If the same fuse blows again, don't consider the system operational until everything has been checked out.