Dangers include (but are not limited to): high currents and associated fire risk; acid electrolyte is very corrosive and rather poisonous; electrode materials are cumulative poisons which can damage the environment.
The positive plate consists of lead oxide on a lead plate, and the negative plate consists of a high surface area of lead. The electrolyte is dilute sulphuric acid. On discharging the following reactions occur:-
Positive: 3e- + PbO2 + 4H+ + SO4- -> PbSO4 + 2H2O
Negative: Pb + SO4- -> e- + PbSO4
Mass of reagents to produce 1 mole of electrons:- (Pb=207.2; S=32.1; O=16.0; H=1.0) 416.4g.
Voltage: 2v nominal; 1.65v discharged; 2.44v charged
Maximum discharge: ~25C at 1/2 voltage
Cycle Life: 300-500 (800-1000 with advanced charging)
Typical failure mode: lead sulphate on the plates turns to an isomer that is not very conductive, and which doesn't adhere to the electrodes well. This process only occurs in a cell that is not fully charged, and is called sulphation.
Dangers include but not limited to: massive energy can start fires, cause burns or cause the car to become uncontrollable; lead and lead compounds in the battery are poisonous, and must be disposed of in ways that are good to the environment; sulphuric acid in the battery is poisonous and very corrosive, and will eat your face off if it spills on you. These batteries will spill if they are tipped over, and so may well spill in an accident. Charging produces an explosive, highly flammable mixture of hydrogen and oxygen, which can cause your car to go the same way as the Hindenberg if it builds up. Also the batteries are heavy, so you can injure yourself handling them, or make your car dangerous by overloading it.
Lead acid technology is the traditional technology for automotive accumulators. And here lies a trap: the normal car battery is designed to give immense current for a short time, and then be recharged - it is not designed to be flattened repeatedly. For an electric vehicle, the ideal battery is called a traction battery. These are designed for both high current and for deep discharge.
Therre are more modern leisure batteries that are also suitable. The problem with leisure batteries is that they are damaged by high current, but more modern designs allow higher currents to be delivered without damage. A similar chain of reasoning applies to ups batteries, which are also designed for deep discharge.
It is a characteristic of lead acid technology that it can deliver enormous powers. Leisure batteries don't like to deliver very high currents (it shortens their life) but traction batteries are very happy to deliver powers of thousands of watts. Even leisure batteries are happy to deliver hundreds of watts. There is a penalty, though: if you take energy out quickly, you get less of it.
Range, capacity, temperature and current are all related.
I think it can be seen that the fraction of stored energy that a lead acid battery will yield depends strongly on two variables: temperature and power. Since the voltage is constant, more power equals more current.
Temperature should be regulated to be between 0C and 25C - temperatures below 0C seriously compromise capacity, and temperatures above 25C can result in corrosion of the battery electrodes by the acid. The temperature gauge should display battery temperature: if a battery exceeds the manufacturer's range the controller should prevent the motors from being connected. This is especially important at high temperatures, where the possibility of thermal damage (or even of fires) arises. The processes of charging and discharging lead acid cells warm them up, and this must be regulated. If the temperature exceeds, say, 50C (but check your manufacturer's ratings) the batteries must be disconnected.
It is also worth remembering that a flat lead acid battery will freeze if the temperature gets too low. In low temperatures, especially overnight, the battery should be connected to the charger, to allow temperature regulation by trickle current. The charger will sense battery temperature and keep it at or above 0C. As a general rule, leaving the charger plugged in overnight is a good plan!
It's worth pointing out that Reserve Capacity is the amount of time a 12v battery will deliver 25A before it reaches 1.75v/cell. That is, the endurance at a rate of 300W per unit. In electric car terms, that's not much. The main use of reserve capacity data is to estimate Peukert's Number.
Lead acid cells generally show a good correlation between amount of charge and terminal voltage, but only after the cells have been given a while to recover from charge or discharge. This gives the possibility of doing percentage fuel metering by observing the off load voltage after an hour or so.
Here is a open circuit voltage to remaining fuel level curve for the Optima battery in an EV application (source:Svein Medhus):
| OCV (Volt) | SOC (%) | DOD (%) |
|---|---|---|
| 13,20 | 100 | 0 |
| 12,95 | 95 | 5 |
| 12,7 | 85 | 15 |
| 12,6 | 75 | 25 |
| 12,5 | 65 | 35 |
| 12,4 | 55 | 45 |
| 12,3 | 45 | 55 |
| 12,2 | 35 | 65 |
| 12,1 | 25 | 75 |
| 12 | 15 | 85 |
| 11,9 | 5 | 95 |
For current integration, Peukert's Number becomes significant, as typical lead-acid cells have Peukert Numbers of 1.05 to 1.25: which means that the discharge rate is signficant. In performing current integrations, Peukert's Number should be applied both to charging and discharging to get an effective current value.
The "classic" way to charge a lead acid cell is to apply a constant voltage and limit the current to provide a maximum power input. For most lead acid arrangements (IE the starter battery in an ICE vehicle) the input current is limited to a low value that cannot harm the battery - typically less than 1C. The voltage chosen affects the total storage. A lower voltage stores less power, but might not result in electrolyte loss, whereas a higher voltage might store more power and result in electrolyte loss through "gassing". "Gassing" gives of a stochimetric mixture of hydrogen and oxygen, which is very, very explosive.
Here is a table of the significant voltage levels for a classic "wet" lead acid cell.
| Battery State | voltage per cell |
|---|---|
| Finishing charge endpoint (gassing - not for sealed cells) |
2.45v |
| Bulk charge endpoint (not gassing - maximum for sealed cells) | 2.3v |
| Fully charged, no load | 2.1v |
| Discharge endpoint (80%) | 1.75v |
| Flat | 1.6v |
A charging method for this sort of cell might consist of an initial charge rate of 0.1C until the cell temperature is between 10C and 25C, followed by a rapid charge current limited to 1C and voltage limited to 2.45v per cell. When the charge current drops below 0.02C the voltage is dropped to 2.3v per cell to preserve the electrolyte, and this voltage maintained until the charger is removed. No initial or finishing charge is used in this regime.
For sealed cells, it is important not to allow gassing to occur: so the bulk charge voltage is limited to 2.3v per cell. Some sealed cells include a catalyst to remove the gas generated by the gassing: in these the bulk voltage should still be 2.3v per cell, but a finishing charge to 2.45v per cell should be applied current limited to say 0.1C. When the finishing charge ends at a current of less than 0.02C, the standby voltage is applied.
Regenerative braking should be at the current specified for bulk charging - that is, at the 1C rate.
You should always check with your manufacturer to make sure that these charging regimes are suitable for your battery!
A report published at the 17th Electric Vehicles Symposium in Montreal, Canada described an experiment performed with the Optima sealed lead acid battery. Basically they used an advanced charging method to maintain battery capacity while doubling the number of cycles available. Since replacing batteries is the most significant cost of running electric cars, that's worth bearing in mind.
They offered two strategies, which they called Zero Delta Voltage Charging and Current Interrupt Charging.
Both charging methods started with a bulk charge to 70% capacity at 50A (for these batteries, at a 1C rate). Then two different finishing methods were used.
Zero Delta Voltage Charging has a two stage finishing charge: after the bulk charge to 70% charge at 50A, then a charge at 10A until the voltage change is less than 15mV per minute for five minutes. Lastly a finishing charge of 10A is applied for between 18 minutes (first 296 cycles), 60 minutes (296-340), and 36-41 minutes for cycles after this. Obviously the second step would need to be done on a per-cell or at least per-battery basis.
Current Interrupt Charging has the advantage that it can be done on the whole battery, but has the disadvantage that it seems to destroy cells. On the other hand, that might be because the discharge continues until the average battery is discharged, not until the first battery is discharged. The method is to use a pulsed overcurrent of maybe 7.5A, at a duty cycle of 50% and a cycle time of 10min. This is done until the off-current voltage reaches the fully charged voltage.
It would be instructive to see if there was an improvement in battery life by using separate discharge voltage monitors. It would also be instructive to see the ZDV method used, since the 2nd step would need to be applied to individual batteries. In an ideal world, it'd be applied to individual cells, of course...
A typical deep discharge cycle will supply the required motor voltages and/or currents until the endpoint voltage is reached, then illuminate the low fuel warning light and regulate discharge current to maintain this low voltage. The low fuel warning light should stay on until the battery is recharged.
Assuming a typical 80% discharge, the last 20% of the battery should be delivered at currents of 6 to 10 amps, which are equivalent to about 30-100W/unit for the Elecsol units described. A 22-unit battery might deliver 660-2200W this way, which will give maybe 20mph on the flat. This might be available for another 200Wh per unit, if the cutoff is at 20%, so the available power might drive another 2-6 hours. That will get another 40 or 50 miles, but at a cost of great battery wear.
The nice man at AVT recommends using "Absorbed Glass Mat" batteries, as he doesn't approve of flooded batteries in cars (a perfectly understandable view), and recommends "group 27" batteries. He also doesn't like the Elecsols on the basis that he thinks they have excessive internal resistance.
Here is some statistics on the Tango car using 25 Optima batteries:-
| Average Miles Driven per Charge | 8 | 16 | 20 | 24 | 32 | 40 | 48 | 56 | 64 | 80 |
| Percent Depth of Discharge (DOD) | 10% | 20% | 25% | 30% | 40% | 50% | 60% | 70% | 80% | 100% |
| Expected Cycle Life (Charge/Discharge Cycles) 100% Depth of Discharge (DOD) = 80 miles |
4,600 | 4,250 | 4,000 | 3,400 | 2,100 | 1,200 | 600 | 400 | 250 | 200 |
| Battery Life in Miles | 36,800 | 68,000 | 80,000 | 81,600 | 67,200 | 48,000 | 28,800 | 22,400 | 16,000 | 16,000 |
Here are those figures rearranged to a general form that applies to any Optima project
| Discharge depth | Number of cycles | discharge storage product |
|---|---|---|
| 10% | 4600 | 460C |
| 20% | 4250 | 850C |
| 25% | 4000 | 1000C |
| 30% | 3400 | 1020C |
| 40% | 2100 | 840C |
| 50% | 1200 | 600C |
| 60% | 600 | 360C |
| 70% | 400 | 280C |
| 80% | 250 | 200C |
| 100% | 200 | 200C |
It is quite clear from this that keeping the discharge level to the 20%-50% region is a huge advantage; and that not going over 70% is a very good idea. Ways exist to help these batteries recover; but better not to get them into that state!
| Manufacturer | Part | Chemistry | Voltage | Capacity | Weight(kg) | Dimensions(mm) | Peak Power | Continuous Power | Cost | Cycles | Peukert Number | Energy Weight Wh/kg |
Wear Cost (per kWh per cycle) |
Notes |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Theoretical | Reagents Only | Lead Acid | 2.0 | 26.8Ah 53.6Wh |
0.416kg | - | 670W | 26.8W | - | 500 | - | 128.8 | - | 1 mole of electrons |
| Elecsol | 80/100 | Lead-acid carbon fibre |
12 | 100Ah 1.2kWh |
19kg | 277x175x190 | ??? | ??? | £84.00 | 500 | 1.15 | 63.2 | £0.14 | May suffer on >50% discharge Flooded |
| Trojan | 27TMH | Lead-acid | 12 | 115Ah 1.38kWh |
26.4kg | 324x171x248 | ??? | ??? | £105.00 | 500 | 1.22 | 52.3 | £0.15 | flooded |
| Optima | D34 | Lead-acid AGM | 12 | 55Ah 0.492kWh |
19.5kg | 254x173x199 | 8.7kW 870A@10v |
8.7kW 870A@10v |
$143.00 | 350 700 3400@30% |
1.04 | 33.8 | $0.48 $0.24 $0.16 |
See advanced charging |
| Optima | 31 | Lead-acid AGM | 12 | 70Ah 0.84kWh |
27.2kg | 326x165x242 | 10.125kW 1125A@9v |
10.125kW 1125A@9v |
?? | 350 700 3400@30% |
1.04 | 33.8 | $0.48 $0.24 $0.16 |
See advanced charging |
| Hawker SBS | SBS-60 | Lead-acid AGM | 12 | 51Ah 0.612kWh |
18.5kg | 220x121x260 | 4.5kW@10v | 441W(1h rate) | £43.95 | 400? | 1.18 | 33.1 | £0.18 | Bargain basement battery |
| Exide Marathon | T12V100 | Lead-acid AGM | 12 | 100Ah 0.612kWh |
37.5kg | 548x115x230 | 10.8kW@6v | 840W(1h rate) | £12.00 (eBay) | 400? | 1.20 | 32 | £0.025? | Surplus stock |
| Hawker Odyssey | PC1700 | Lead-acid AGM | 12 | 70Ah 0.84kWh |
27.2kg | ?x?x? | ??? | ??? | $200 | 400 | 1.06 | 30.9 | $0.60 | Good Peukert - lots of lead! |
Although they are probably the most popular batteries for electric vehicle projects, they are not really the most suitable. Lead acid batteries are poisonous, corrosive and environmentally damaging, and the most efficient designs (the flooded cells) are likely to spray sulphuric acid all over the scene of an accident.
Lead sulphate cells are not good at low temperatures: at anything below freezing the capacity falls sharply.
What is more, lead acid cells suffer from something called the Peukert Effect, where rapid discharging leads to less total energy being removed from the cell. This can easily contribute to a loss of 25% to 50% of the total energy available for the vehicle.
Future development will not get very far with the lead acid cell. The theoretical best storage is 128.8Wh/kg, and existing cells (the carbon fibre batteries made by Elecsol) get to nearly 50 percent of this. Even AGM systems (for example, the Optima) are more than a quarter of the way there. It seems unlikely that any lead acid battery for sale that stores more energy than today's battery. It might be safer, and it might last more cycles, but that is all.
This page is part of an Open Source Electric Car Project, and is written and maintained by Simon. At this stage these pages are constantly under revision. Thoughts and comments are welcome.