Batteries For Models
Batteries For Models
Safe Disposal
Before I offer any information I wish to emphasise that
UNLESS YOU ARE PREPARED TO TAKE THE TROUBLE TO CAREFULLY STORE, AND IMPORTANTLY DISPOSE OF, UNWANTED BATTERIES THEN I RECOMMEND
AVOID USING ANY BATTERY UNLESS ABSOLUTELY NECESSARY - whether rechargable or not !
This may sound a bit harsh but batteries easily have the potential to cause burns and/or fire and certainly great damage to the environment if not disposed of responsibly. I have made many complaints to local councils and clubs because of their cavalier attitude to them which reflects the same attitude in society. We are all aware of the brain damage caused by lead which thankfully now has been removed from petrol yet the same still has not happened to Cadmium and other elements common in batteries and their harm is many orders of magnitude worse.
Frankly for environmental reasons I generally do not recommend the use of batteries anywhere where safe cabled electricity can be made available.
I AM NOT IN FAVOUR OF USING BATTERIES GENERALLY
GENERAL CHARACTERISTICS
Rechargable Batteries
The general characteristics for comparison are:
State Of Charge & General Battery Characteristics
My dislike of batteries over I’ve had many requests for simple information on how to determine the state of usefulness of non-rechargable dry cell batteries. Fortunately their open circuit (no load) voltage is a good indicator. So by removing them from a device and applying a digital multimeter (very cheap these days) one may ascertain the ‘life’ left in the battery by using the following table.
Some devices are designed to run for a very long time before they run the battery flat - notably clocks and remote controls. Therefore their circuits can still continue to work at unusually low voltages so even if your battery nolonger powers normal devices you can put almost expired batteries in these devices and they will continue to work - more bang for your buck ! I find it very frustrating if a remote control suddenly stops working so to reduce how often this happens I actually don’t do this with them but, hey, you may be happy with this.
* Beware Many babies and children have been badly injured by swallowing these.
Non-rechargable Batteries
The general characteristics for comparison are:
Rules Of Thumb
A battery at low state of charge exhibits a low terminal voltage which increases as charging occurs. When plotted on at graph the relationship is a curve (see below). The same in true for discharge.
A simple power supply (normally constant voltage) applied to a discharged battery will initially sink a lot of current which steadily decreases as charging, and hence the terminal voltage, increases. In which case the battery will at first heat up dramatically and local hot-spots could cause an explosion or at very least premature cell failure. Hot is anything above warm to the touch (30°C max). Therefore charging current must be limited -cheap lead-acid chargers rely on the use of high impedance transformers to perform this limiting.
In order to prevent overcharging, and therefore sometimes irreversible damage to the battery chemistry, the amount of charging supplied must be limited. This is done by limiting the charge current and the time that the current flows for (see below).
These two limits indicate that a simple power supply eg a model transformer, is not sophisticated enough.
Example Calculations (Lead-Acid)
Temperature Effects
Sadly temperature affects all batteries greatly. These figures are based upon use over 15-25°C. If your use of the battery is very far away from 20ºC then the voltages below should be adjusted at a rate of -0.022v/ºC.
Vmax -charging
Vnorm -charging
Vmin -discharging
Imax -charging
Imin -trickle
Imax -dischargimg
Norm Temp range:
Lead Acid
2.6V
2.4V
1.8V
10-30% *
3% *
80%*
-10 to +50ºC
Nickel-Cadmium
1.4V
1.35
1.05V
TBA
0 to +50ºC
Lithium-MetalHalide
1.66V
1.66V
TBA
0 to +50ºC
Percent of normal capacity in Ah. For example 10% of 15Ah is 1.5A.
Battery Basics
Recharge
Continuing with the example of a 12v Lead Acid battery I will now show how simple it is to calculate what one must do. Say the capacity is 5A.h. In days of old it was normal to recharge from empty to full capacity over a 10hour period. Thus:
Recharge current must be: 5 A.h / 10 hours = 0.5A
However all the specifications I have looked at and I have found from experience that there is no harm done (nor no significant temperature rise) if one charges at at least 30% higher than the 10hr rate for 30% less time (or proportions of this pro rata). I sell batteries at this exact capacity and always recharge them at 0.7A and just take a measurement with a digital multimeter (after settlement) every half hour BUT don’t forget and leave the poor blighter to “overcook” otherwise you may have a small or large catastrophe on your hands or your carpet !
There actually ought to be a small (5%) overcharge built in to take inefficiency (hence battery warming) into consideration. However if one doesn’t do that then one tends to undercharge slightly and avoid the risk of overfilling.
Websites
The following websites I’ve found to be useful:
Battery University -a full description of all battery technologies
MORE INFORMATION
Charging Limits
Estimating Charge
The units of battery charge capacity are Amperes x hours abbreviated A.h. A small battery may have a capacity 1 A.h. However written on it may be 1000 mA.h. which is the same.
In order to neither over, nor under, charge the battery an estimate of the total additional charge to add to a battery needs to be made.
For practicality batteries want to be charged as quickly as possible but they will heat up and may be damaged by doing this too quickly, therefore a charger must limit both current and time.
Physical Construction
The simplest form of battery is a single cell. Battery chemistry means that any single cell has a terminal voltage of just a few volts. We are all familiar with rechargeable, and all those awful use once only, batteries which are normally cylindrical eg torch batteries.
Real world applications require higher voltages therefore batteries are wired in series (never parallel). Often these are supplied in a package. The packages vary greatly in size and shape but often are square in overall outline eg car batteries.
The lifetime of batteries is limited and therefore they must easily be disconnected and reconnected. Terminals are often studs, screw threads or non-reversible sockets.
BATTERIES - AN EVER PRESENT DANGER
Chemicals
I have already mentioned that I am NOT in favour of the use of batteries generally (see first paragraph). However our portable world requires them. The chemicals inside vary from being seriously corrosive to downright toxic:
Do NOT: break open, incinerate, or carelessly put any battery into the environment. Please return to supermarket or council depot.
Electrical Wolf In Sheep’s Clothing - FUSES MUST BE USED !
Also the innocent low terminal voltage of batteries belies their serious and ever present danger. I know a great many people who have had to rewire their entire models because they failed to see that a simple fuse must be connected as close to the battery as possible. Due to the very low internal resistance of batteries huge currents will flow if a battery is short circuited. Burns and fire are an obvious consequence. It’s not rocket science:
ALWAYS: Place a fuse, and preferably an isolating switch, in series with your battery.
Charging
Example Voltage Characteristic (Lead-Acid)
Graph
Taking an example of a 12v Lead-Acid battery the graph below can be drawn. From it one can see:
a) The terminal voltage increases with charge.
b) The characteristic, whilst usefully not steep, is essentially linear between 10% and 90% capacities so we may judge the state of charge easily at any time.
c) Outside of these limits the graph is steeper.
d) The red area indicates that damage can occur if the battery is discharged below 8% charge.
A Note On Measurement
Please note that these measurements are taken NOT immediately after significant charge or discharge. It is often desirable to take a measurement during charging. If this is the case the battery must be left for 3-5mins to settle. A light discharge, through a small lightbulb for example, is helpful to speed up this settlement.
Avoid Full Discharge If Cells Are More Than A Year Old
Full discharge, whilst in itself is within the design specification of any one cell, is not advisable in multiple cell configurations. The reason is not obvious but will now be put forward. As the life of the cells proceeds they begin to age, that is they become less efficient at giving out all of their charge, and more detrimentally some may hold their charge for longer than others. That means that during discharge some may completely run out before others. However the others will continue to pump out their charge. The load that they are supplying makes a continuous circuit (obviously) but this may mean that those cells that ran out may become reverse charged by the better ones. This even more quickly shortens the life of the poorer ones.
Conclusions
The useful life of the battery depends on how kindly it is treated. Indeed Lead-Acid batteries are tolerant of high discharges but Lead-Acid batteries are not keen on being completely flattened and especially not left that way. Apparently a layer of Lead Sulphate builds up of the plates effectively reducing their surface area.
Therefore stick to the range 11.3v to 12.7v and all will be well.
Disclaimer
Whilst some care has been taken to check externally linked websites no responsibility is offered nor implied for the suitability, legality or reliability of content therein.