When I started reading that “car batteries are never charged above 70% capacity”, I had trouble believing it for several reasons
More accurate statements about charging a battery in a car would be that the battery will not charge fully if -
I) You install a battery needing a high charge voltage (Calcium-Calcium needs 14.8 volts)
ii) Your Alternator or regulator is faulty.
iii) Your Regulator Alternator is an older design that puts out less than 14.2 volts.
iv) You don’t drive long enough to replace the charge taken from the battery (charge time for a discharged battery can range from 2 to 12 hours). www.absorbedpower.com/battery/support/applications/ www.batteryuniversity.com/partone-13.htm
v) You add heavy loads (winch, high-power audio,
HF Radio) and your Alternator can no longer provide the power.
a. Does it really matter if your battery is charged to 70% or 95% ?
If you just use your battery for Starting in a reasonably warm climate and the battery is in good condition, then it doesn’t matter if you have 70% or 95% charge (ignoring sulphation damage) because starting only uses up between 2 and 5% of battery capacity.
But if you are going into remote areas and/or you are using the battery to power fridge, lighting, winch etc then you really need to have lots of battery capacity. If your alternator is in reality only charging to 70% then you might be looking at spending good money on better charging, better Isolator or bigger batteries - but if your Alternator is in fact charging close to 100%, this would be wasted money that you could be spending differently to get a real improvement.
If you increase your battery charge level from 70% to 100%, you are more than doubling your available capacity because you should never regularly discharge any battery below 50% (yes, even DEEP cycle) so discharging from 70 to 50 gives you 20% available but discharging from 100 to 50 gives you 50% available.
b. These are articles that suggest “car batteries aren’t charged above 70% capacity”-
1. “With common machine sensed alternators, batteries are, in most cases, charged to only 60 - 70% of their capacity, however long the engine is running” www.adverc.co.uk/technical/technical-altmis.asp This article states you will get closer to 100% charge if you buy ADVERC’s special regulator !!!!
2. “once a battery has been on charge for more than about 1/2 hour, changes take place . . . . which have the effect of increasing the internal resistance. This build up of internal resistance will inhibit the flow of charging current, . . . . It is estimated that batteries recharged in this way never reach more than about 70% full charge.” www.acornengineer.com/kestrel_alt_controller.php This article states you will get closer to 100% charge if you buy Kestrel’s special regulator !!!!
3. “In practice, starter batteries spend most of their life somewhere between 65% and 70% of full charge” www.caravanandmotorhomebooks.com/articles/lead_acid_batteries.htm
4. “Vehicle alternator/regulators . . . work this way and . . . charging tapers almost to zero once the battery is 70% 'full' “. www.exploroz.com/vehicle/electrics/solar.asp
5. “Car batteries are seldom charged above 70% of their capacity, but they are designed for shallow discharge.” Deep Cycle Charger - Silicon Chip, page 34, Nov 2004 (SC) www.siliconchip.com.au/cms/A_103239/article.html
6. “Vehicle charging systems stop charging when batteries are around 70% 'full' “ www.absorbedpower.com/battery/support/faq/
7. “Standard car and truck alternators are designed, for various good reasons to recharge car batteries to only 70-80%”, www.fridge-and-solar.net/agm.htm
8. “idling the engine or short stop-and-go trips during bad weather or at night will not fully recharge a car battery” - At least this one states the conditions under which a car battery will not be fully charged. www.uuhome.de/william.darden/carfaq14.htm Sect 14.2
c. Possible explanations for the 70% Charge Myth
1. These statements may have been true in the days of 30 Amp Generators with Relay regulators giving varying voltages. But with today’s high-capacity Alternators, Electronic Regulators, Temperature Compensation and remote Battery Voltage Sensing, overcharging is more likely. Overcharging would boil a battery in weeks whereas undercharging would cause a slower death by sulphation - since Relay Regulators could not control voltage accurately, they chose to undercharge rather than risk overcharge.
2. “During the first Stage of charging the battery is charged to 70%-80%” is widely accepted www.batteryuniversity.com/partone-13.htm , Somehow this leads people to erroneously believe the simple Constant-Voltage charging in a car can only provide the first 70% of charging in Stage 1 (current-limited) charging mode. To understand the rest of this article, you really need to understand the basics of Lead-Acid Battery Charging, so I suggest you read this reference.
3. References 3 and 4 are using the statement that a normal Alternator only charges to 70% to convince you to buy their improved Alternator Regulators. References 6 and 7 use this statement to convince you to buy their AGM batteries rather than standard wet-cell batteries.
4. If you connect a battery that has been charged in a car to another charger that produces a higher voltage, then current will flow into the battery. But people assume that this current is increasing the charge level of the battery, and therefore the battery was not fully charged, but this is not necessarily true. By the "conservation of energy" principle, the electrical energy flowing into the battery must go somewhere, and for a Lead Acid Battery there are three main options -
i. Converting the chemicals to store energy for later use during discharge.
ii. Heating up the battery.
iii. Electrolysis of the water, visible as gassing.
If 1 amp is flowing into a fully-charged car-size battery and all this energy is converted to heat, there will be a barely noticeable temperature rise of the battery.
d. The incorrect assumptions
But in fact a car actually provides only Stage 2 Constant Voltage charging (unless your Alternator is seriously under-rated), NOT Stage 1 constant-current charging !!!!. Stage 1 isn’t provided for in a car because starting the engine only uses 2 to 5 % of the capacity and Stage 1 current-limiting is only necessary for batteries that have been discharged to below 70% remaining.
So if you have a deeply discharged battery (Starting or Deep Cycle) connected to a Car Alternator. it won’t be charged in Stage 1 mode (current limited to a level safe for the battery) but rather it will charge at whatever current the battery will take from 14.4 volts - this could be 50 amps initially. So instead of taking 2 to 5 hours to get up to 70% in Stage 1, the battery is fast charged is much less time (and shortening the battery life). Charging current will continue to taper off as is normal for Stage 2 until the battery is fully charged. Also a car Alternator never switches to the lower (Stage 3) Float Voltage, (except due to compensation from temperature rise) so charging does not even slow down dramatically when 90 to 95% charge is reached, as happens in a 2 or 3 Stage charger.
So this following quote is wrong in concluding that charging tapers to almost zero when charge reaches 70%. In a car, charging actually tapers to almost zero when charge reaches 95%, but does not reduce to float voltage (that is why Taxi owners go to the expense of relocating the battery to the boot, because modern Alternators will over charge on vehicles driven on long shifts). “If the charging voltage is fixed, as the battery charges and its voltage rises, charge rate automatically falls. Vehicle alternator/regulators . . . work this way and . . . charging tapers almost to zero once the battery is 70% 'full'. This prevents overcharging with vehicles like taxis on shifts”. www.exploroz.com/vehicle/electrics/solar.asp
The graph here shows clearly that current tapers to zero at the end of Stage 2 (95%), NOT at the end of Stage 1 (70%) - www.batteryuniversity.com/partone-13.htm
All modern car charging systems use remote battery voltage sensing
http://www.autoshop101.com/forms/h8.pdf and ambient temperature compensation, so you would be wasting money to “change” to one of these “improved” Alternator Regulators.
e. Why is it obvious that modern Car batteries are charged to near 100% ?
The average car battery lasts four years and a battery that is only ever charged to 70% maximum will Sulphate very quickly and therefore will not last 4 years.
From www.uuhome.de/william.darden/carfaq.htm -
- Section 13.2 - "sulfation will begin occurring when the State-of-Charge (SoC) drops below 100%"
- Section 14.2 - "Generally, idling the engine or short stop-and-go trips during bad weather or at night will not fully recharge a car battery. When a dead battery needs to be recharged, it is best to use an external battery charger because you could over heat and damage your vehicle's charging system and your will save a lot of gas and wear and tear on your engine". But this does not mean that long drives won't fully charge a battery, or that an adequately sized Alternator will not safely charge a partially discharged battery.
“Recharging less than 100% may result in premature battery failure” “loss of capacity can become permanent if the battery remains in a discharged or undercharged state for even a short period of time” - www.absorbedpower.com/battery/support/
h. Is my Alternator big enough to charge my battery ?
The general guideline seems to be that the Charging current should be no less than 1/3rd of the battery capacity - so for a 125 Amp Alternator (assuming 25 amp is needed to run the loads) you can charge up to a 300 AmpHour battery. www.adverc.co.uk/technical/technical-teched-5.asp#Battery%20Facts
“A simple
test to determine if the charging system is large enough is to
check the battery's State-of-Charge after the surface charge has been removed. If the State-of-Charge is consistently above 95%, then the charging system is fully recharging the car battery based on your driving habits and electrical load.” Sect 5.3 from www.uuhome.de/william.darden/carfaq.htm
i. How can I be sure what the level of charge is ?
Measure your battery voltage early in the morning without ANY load connected to it (NO interior light on etc) - is it 12.3 volt (70% ) or is it closer to 12.6 volt (90%). Here are two opinions on using voltage to determine capacity..
“Instantaneous measurement of deep cycle battery voltage is meaningless and often leads to quite wrong conclusions: all you can really be sure of is that the meter's working.” www.exploroz.com/vehicle/electrics/solar.asp
“The state-of-charge of a lead-acid battery can, to a certain extent, be estimated by measuring the open terminal voltage. Prior to measuring, the battery must have rested for 4-8 hours after charge or discharge and reside at room temperature.” www.batteryuniversity.com/partone-13.htm
To be really sure of charge level, measure the SG of the battery - messy, but much more accurate than voltage measurements. It is still important to wait several hours after the last charge or discharge before measuring SG. The actual SG that indicates a full state of charges varies for different battery types, so you may need to “calibrate” for your battery by float charging it for a while. Here are some standard figures from www.uuhome.de/william.darden/carfaq9.htm -
1.260-1.270 for wet Starting batteries with pasted plates
1.267-1.284 for wet motive Deep Cycle batteries with solid lead positive plates
1.300-1.310 for wet motive deep cycle batteries with tubular positive plates
Of course if you have Gel or AGM Battery you will have to use Voltage Measurements because you can’t measure SG. There are some Capacity meters based on pulse load testing, but these meters cost more than a battery.
More complex methods based on measuring the battery internal impedance exist but the equipment is expensive and needs to be calibrated for specific battery types.
i. Don’t confuse State of Charge with Available Capacity
If you bought a 100 Amphour battery four years ago it may
well show 100% remaining capacity on voltage and SG tests, but that doesn’t mean you have 100 Amphour available. As the battery ages its available capacity reduces, so it may now be a fully charged 40 Amphour battery. The only way to verify Available Capacity is to do a discharge
test.
j. So before deciding on what you need to do, to have your battery charged above 70%, do some simple tests to see if anything really needs to be done at all. Changes should only be needed if your car is very old, has a very large electrical load or has a fault.
Mike DiD