This is a highly specialized field and not ever mastered by those that ‘dabble’, as some 4×4 equipment retailers and off-road trailer manufacturers do. Boy, have I heard some rubbish advice being shared by some 4×4 equipment retailers on this one! There are two types of battery applications that concern us: ‘Float’ and ‘Cyclic’. Typical of a float application is an ordinary car battery, where once the vehicle has started the current is replaced quickly as the vehicle drives. Cyclic on the other hand is when a battery is charged and then used with no or little charge being replaced. Such an application is common to the 4×4 scene when a vehicle arrives at a destination with its battery in an almost full state of charge. The fridge and lights run through the night and in the morning the battery has lost a significant amount of its charge, When selecting the type of battery, ask what is the application, float or cyclic? If a calcium battery (float application) is used in a cyclic application and the battery is not able to be recharged immediately, the battery will sulfate, causing irreparable damage to the plates. An apparent loss of capacity is noticed and after a short while total failure results. Should a battery designed for cyclic application be used in a float charge mode the result is stratification of electrolyte, mossing of the plates and a large amount of active material falling off the plates becoming sediment. This sediment eventually causes an internal battery short-circuit which cannot be reversed.
Battery charge and temperature
A battery charge is also affective by temperature. A rule of thumb for this follows: A battery is rated at 25°C; for every degree below 25° the battery will lose one percent of its capacity. Its life however will be increased (before failure). Also for every one degree above 25°C the battery will gain one percent of its capacity but its life will be reduced.
Duel Battery split-charging systems/ Battery isolators
When a freezer or lighting is powered from the vehicle’s primary battery, there is a risk that it will be flattened overnight or during an extended stay. Should this happen in the bush the vehicle may have no way of being started. Duel battery split-charge systems solve this problem by enabling a second battery to run the fridge and lighting while the vehicles primary battery remains unaffected. This second battery must be able to cope with the cyclic nature of the application. Deep-cycle and high-cycle are designed to cope with the larger discharge and recharge cycles than normal vehicle batteries.
current draw and recharge
If you are choosing between a number of popular makes of freezers and judging them by current draw then I would suggest you buy any one and focus on the real issue: How is the current going to be put back and how long is it going to take? This is the big question as there are always difficulties experienced with recharging deeply discharged batteries, and there seems to be no cure, just bad , good and better solutions.
Charging Deep-cycle batteries
Lead-acid batteries, be they float or deep-cycle types, have recharging characteristics that can frustrate the user. Because deep cycle types are used in many off-road applications, I will deal with these alone. When a deep-cycle battery’s charge drops below about 11.8 volts it resists accepting a charge. No matter how much current is fed into such a battery it can appear to be lifeless. The reason appears to be associated with voltage. Ordinary car alternators produce between 13.8 (e.g. Bosch-English & German vehicle) and 14.6 volts (e.g. Nipon Denso- Many Japanese vehicles.) it is not enough! This is because when the battery voltage is low, when the charge is initialized, unless the charge voltage above 14.6 volts, only a small amount of current is accepted by the battery. After some hours or charging, the battery voltage slowly increases, and in doing so, Once the battery is in a state of about three-quarters full, its voltage is enough to-receive all the current it needs. One such battery recharge system is under development.
Take a look at the following typical scenario: A battery is used cross country all day. It reaches a point when the engine is shut down for the night and the fridge and some lights are turned on. The following day the vehicle remains stationary. By the morning of the second day, by the morning of the second day, two nights and a day have gone by. The daytime temperatures are high and the fridge has been running about 70% of the time. The operator knows that the battery charge must be getting low but he or she is not too worried because there is a duel battery split-charge system fitted. He or She decides to take the vehicle for a short run, mainly to charge the auxiliary battery. The battery voltage although high enough to keep the freezer working has dropped off the high current accepting plateau, ±11,8 volts’. The vehicle is driven for a two hour game drive; plenty of time so the driver thinks, to recover the battery with the 100-amp alternator fitted. But, over this two hour period the deep-cycle battery has accepted half an amp for the first hour, one amp for 30minutes and 20 amps for the last 30 minutes- a total charge of 10 and a bit amps. However, the operator is under the false impression that he has fully or almost fully-charged battery. Night falls and on goes the electric lights while the freezer continues to keep its contents frozen. By twelve that night the freezer low-voltage cut-out activates and in the morning everything has thawed. The operator is baffled and curses the battery supplier because he thinks he has been sold a bad battery.
Time required for a flat battery to accept a measurable charge
on charge voltage Hours
16 volts : Up to 4 hours -check every half hour
14-15,9 volts : Up to 8 hours -check every half hour
13,9 volts: Up to 16 hours -check every half hour
The table above indicates the time required for deep-cycle Delco battery to receive a measurable current and the usable power of two models of the Delco voyager. For standard use, discharge is from 100% down to 50% charge. In emergency use, the table indicates usable power from 100% to 0%. A second battery wired in parallel will double the value. (excluding reductions due to battery mismatch.)
Battery split-charge/ recharge solutions:
- Auto-relay. E.g. Gemini. More expensive, automatic, efficient.
- A great big heavy duty switch. E.g. Marine type switches. Simple, inexpensive, reliable, subject to user error or forgetfulness.
- Diode-based battery isolators. Simple and inexpensive but so inefficient that they are not worth considering.
Auto-relay split-charging systems
These systems, by far the most complicated, charge all batteries in the circuit and with a monitor unit fitted, tell the user the state of each battery. those that read just the voltage are pointless and need to read in increments of one-tenth of a volt. these systems do not change the voltage so do not solve the recharge problems discussed. they are fully automated, which is a plus. their efficiency varies and some are very inefficient. for example one of the better unit work like this: Input voltage in this case is 14,1-volts from a Toyota Nipon Denso alternator, one of the most efficient. Voltage through transistors and other components drop 0,3 volts. loss through wiring and connectors, another 0,5 volts. Voltage to the battery is 13,3 volts. Not even close! Result: the battery is never fully charged and this is a best case scenario! even the most highly inefficient systems only deliver about 13,2 volts to the battery. The battery doesn’t stand a chance of delivering at rated current or being able to accept current at the rate a vehicle alternator can deliver it.
High-current manual switches
Marine switches are switches able to carry the heavy charging currents produced by alternators running at full revs, sometimes over 100 amps. They connect the batteries in parallel so when the alternator charges the one, the auxiliary battery gets the same charge. There is little voltage drop, as long as the cables are thick enough and the connectors good. On the down side, if one of the two batteries is bad, it will discharge the other and if the operator forgets to switch the main battery off at night when the freezer is running, there is the risk of flattening both batteries.
Constant voltage chargers
The idea behind a 12-volt to 12-volt charger is that no matter what the engine speed, the charge voltage remains constant. A constant range of charge is very good for the battery and the battery’s life is extended. Their downfall is that when the alternator is running at full charge and the battery is in a position to accept such a charge, it will only accept the pre-set current.
Diode-based battery isolators
A diode is a component in a circuit that permits current to flow in one direction only. between two batteries it permits one battery to charge but not discharged. This sounds ideal but for inefficiencies of diodes. The voltage drop across a high-current diode is often over a volt. They also tend to permit other current loss, although I am not sure how and why. All I know is not to trust them.
Tips to better battery charging:
- Use heavy cable, solid-crimped connectors (Not soldered).
- Make the cables as short as possible. For every one meter of cable length, the core diameter must be one millimeter. For example: Three meters of cable should have a core diameter of 3mm.
- Set the freezer of switch off at now lower than 11.8 volts if you can.
- Never put a battery in a trailer without the biggest connector you can find. Don’t even consider the tow-hitch connector as the voltage drop is too high. I once measured a trailer battery charge voltage that topped out at 12,2 volts: that is the voltage of an almost flat battery.
Keep it simple
Because this is a complex subject, and few understand it completely, when selecting a system, my advice is to keep it simple.
Deep-Cycle and High-Cycle Batteries
Because Delco is the most popular auxiliary battery found in 4x4s, here is some advice for their use.
Delco Voyager is of flooded cell construction, fully sealed and requires no topping up. The only maintenance required is cleaning and greasing of the connectors. The built-in hydrometer allows an easy check of the state of charge. The battery is often not suitable for use as a regular vehicle battery as its cold-cranking current is often not high enough to start big diesel engines.
- Green: Above 70% charge: Ready for use.
- Black/invisible: Between 50%-70% Recharge if possible. Red: Below 50%: Recharge immediately.
- Yellow/clear: Electrolyte level low. Do not charge.
- Built-in battery hydrometers are a guide only and regularly malfunction or get stuck.
Normal charging requirements
Optimum battery life will be obtained if a green hydrometer condition can be maintained and batteries should never be left in a deeply discharged state. If the state of charge has reacted 100% charging should only be continued for long periods at a reduced rate to prevent long-term electrolyte loss. On-charge voltage should be 13,5-13,8 volts.
Care of batteries
- Deep-cycle batteries are sometimes suitable for normal vehicle use as well as discharging up to 70% of their capacity.
- Keeping a battery cool, keeping it charged and not over draining it are the three most important principles in extending the life of a normal lead-acid or deep-cycle battery.
- Overcharging causes grid erosion and can seriously diminish the ability to accept a charge. A current taper with timer or a suitably controlled regulated voltage is the best protection against overcharging.
- Do not fast-charge a battery, unless in an emergency, especially if it is a deep-cycle type.
Batteries do not store well. When operating a low mileage vehicle or a vehicle that stands for long periods, make sure that the battery is kept in a good state of charge, otherwise it will deteriorate rapidly. Check and top up the electrolyte and recharge every three months- leaving it longer will damage the cells. If necessary store batteries indoors to prevent the electrolyte from freezing as in most cases this destroys the battery. Batteries must be fully charged beforehand and must be disconnected from all loads, however small.
Inverting current from 12-volt DC to 220-volt AC is done with an inverter. new technology has made these devices very compact and virtually indestructible. Overload them and they simply shut down or wire them up incorrectly and they simply refuse to work. for one year I used a solar panel to charge a battery which by means of a 200-watt inverter ran an Apple Mac and printer in ambient temperatures over 40°C. Much of the work on the first edition of this book was done at this time. The inverter became so hot that it could not be touched, yet it operated faultlessly. Current draw reached 10-amps at 12-volts(120-watts). Quasi-sin-wave inverters are suitable for computers, printers, televisions and hi-fis etc. Sin-wave inverters are required for scientific equipment but are unnecessary for most applications.
Despised by all those who work hard to get into the bush, away from noise and stress, portable generators are an unforgivable annoyance. Because they can be easily replaced by alternative power sources that are silent, more ecologically friendly and cost no more, I see no reason why these are used, except in cases where life support systems require high electric current. Running a TV is no excuse: why not then stay at home? As a result, I see no place for portable generators in this site.