DOE Test Methodology for 

Battery Chargers

Issues with testing using lead acid batteries
Marine and RV battery chargers differ from power 

tool and small appliance chargers

DOE Testing assumes all variables are known – battery chemistry, battery size. This is 

not the case in Marine and RV applications.

• The battery charger manufacturer has no influence on the selection of batteries.

• The battery charger could be used to charge a single battery, single battery bank, multiple 

batteries or multiple battery banks

• The dominant batteries in these applications are Lead Acid

 Lead Acid batteries are designed and rated for slower discharge rates than the test methodology in 

the standard (Less efficient at rapid discharge rates).

 Few Lead Acid batteries are designed for full discharge, and those are typically less efficient at rapid 

discharge.

 Charge and discharge rates have a profound impact on measurable capacity.

 Final charge can be a slow process (Limited by diffusion of fluids in porous materials) which results in 

active charging while the test is supposed to be in a float/maintenance mode. 
Test Method should be modified to reduce 

errors introduced by Lead Acid Batteries

• Lead Acid batteries are sensitive to charge/discharge history

• Batteries “as received” will have received different care between 

manufacture, transport, storage and delivery. Seldom arrive fully charged.

• Lead Acid batteries are designed and rated for slower rates of 

discharge than in the DOE test. Typical ratings are for discharge over 

20 hours.

• Faster discharge leads to less predictable energy recovery

• Rapid discharge results in lower efficiency than targeted in real applications

• Lead Acid batteries can have a very long, slow final charging period

• What is measured under DOE test protocol may include active charging as 

well as maintenance current. This distorts the maintenance data.
Optional Battery Preconditioning

• Instead of making the standard condition “as received,” allow 

optional preconditioning to full charge under controlled conditions on 

site.

• Such preconditioning can include multiple cycles of charge and 

discharge, including use in prior tests of the Equipment Under Test or 

in other tests, up to the useful life of the battery.

• If reused for multiple tests, battery use should be discontinued if it 

shows signs of aging such as loss of capacity, higher impedance or 

higher maintenance current.

• Laboratories should set their own quality control to determine fitness 

of batteries.
Optionally, allow longer discharge rates for 

lead acid batteries.

• Batteries are rated for discharge over 20 hour period

• Battery efficiency is degraded significantly at faster discharge rates

• Best practices in the marine industry recommend battery bank sizes 

for “house batteries” to able to handle loads for 24-48 hours with a 

discharge from about 85% SOC to 45% SOC. (only using 40% of 

battery capacity)

• 40% in 24 hours = average discharge rate of C/60 average

• 40% in 40 hours = average discharge rate of C/100
Lower Discharge Rates are Superior

• More repeatable and accurate results

• Less energy can be extracted at high discharge rates

• How much less depends on an exponential function whose result is difficult to 

predict. (Puekert’s equation describes phenomenon, but exponent is battery 

specific and hard to obtain or measure.)

• 5 hour discharge determines a significantly lower Ebatt figure than should be 

achieved in real applications.
Charge and Discharge Curves Data from Home Power – 1989

Typical curves for charge and discharge of 

Lead Acid Flooded Golf Car batteries used 

in off grid installations.

Notice 

1. Charging at high rates requires 

elevated voltage.

2. Discharging at high rates causes 

considerable voltage drop.

3. Difference between charge curves 

and discharge curves is a loss in 

efficiency caused by the battery itself.

4. Nothing in charger design can alter 

the battery characteristics.

5. Choosing battery sizes to allow low 

relative rates of discharge can 

improve efficiency of the system.
Discharge Efficiency 

• Average Loss for flooded lead acid batteries with total discharge at 

C/5 is 7.35%*

• Average Loss for flooded lead acid batteries with total discharge at 

C/20 is 1.8%

• Average Loss for same flooded lead acid batteries with partial 

discharges at C/40 estimated to be 1% - emulating application.

• Differences between C/20 and C/30 estimated to be less than 1% 

difference, reducing sensitivity to actual rate during test.

• Could simplify testing by eliminating repeated testing to hit a narrow time 

window.
0

2

4

6

8

10

12

14

16

0 20 40 60 80 100 120

% Loss During Discharge

State of Charge

Battery Discharge Losses at Different Rates

C/100 C/20 C/10 C/5 C/3

C/5 Discharge has 7.4% Loss

Partial Discharge C/20 has 1.3% loss

Partial Discharge C/100 has 0.8% loss

Based on data from Richard Perez 

in Home Power - Aug. 1993
Maintenance condition may not be obtained 

in 19-24th hours of charge cycle

• Unlike many other batteries, Lead Acid charging cycles can have a 

long, slow tail to final full charge

• Rates are limited by diffusion into porous material and equalization of acid 

density

• If active charging current is still flowing, and not just the self 

discharge replacement current, we will overstate the maintenance 

mode power.

• We have observed the phenomenon in the lab, but do not have a 

complete study to characterize it.

• Simple test to extend the test until the current into the battery is 

stable and reaches an equilibrium.. That might be more like 48 hours.
Extend test to determine Maintenance Mode

• Should be able to characterize phenomenon with a little dedicated 

lab work.

• Some data to be available by end of the month

• Tentative solution:

• Extend test duration to point that maintenance mode power can be 

determined, probably 40-48 hours.
Summary of Recommended Changes to DOE Test 

Methodology, options for Lead Acid Batteries

• Generally same instrumentation and test sequence.

• Allow Preconditioning at Tester and Charger Manufacturer’s option.

• Extend Discharge time to target to 20 to 24 hour window, at option.

• Extend duration of test charge cycle till charging amperage is stable

• Existing methodology is optional on these points if test batteries 

allow for adequate testing under existing standard.