12 volt battery installed

When I bought my batteries I ordered 5 additional as spares. Rumor has it that the failure rate is about 1 in 30.
I have used one of the spares as the 12 volt battery for the accessories. To fasten it I bought some 6mm threaded rod (600mm zinc plated for $2 from Hardware and General) .
I cut it to length and bent about 30mm on one end 90 degrees. The bent end goes through a 6mm hole drilled in the base where the original battery went.
The battery is sitting on a rubber mat. The threaded rod and the hold down clamp are covered in heat shrink to prevent any possible abrasion.

All battery interconnects are installed and wiring done.

This weekend I installed the battery interconnects. They are made from copper sheet, cut into 12mm strips and drilled. All the straps are n shaped so they will allow a little battery movement. This is to prevent the battery terminals from working loose.

The cables are 16mm^2 . Overkill but free. The contactors along the side are double pole. These are used to break the battery bank into 12 x 48v blocks and on 24v block. This makes it save to work on. I tested each contactor separately with my cable shears at the ready in case I had made a terrible mistake. Thankfully they were not required. Across any contactor there can only be 48v max so it is safe to work on. The stainless steel battery clamp strips are covered in heat shrink to give as much insulation as possible. Each battery has the zener-resistor balance circuit connected across.

Next step is to put the +- 300v cables and interconnects to the VFD into orange plastic tube. and begin connecting the controls. (throttle, reverse lights, direction switch and VFD display. Then the chargers and fuse holders for the chargers.

Batteries Installed

Today I installed the batteries and cut and drilled copper bus bars. Also put heatshrink.

I need some stainless steel bolts to attach the bus bars and the balance circuits. The screws and nuts that came with the Greensavers are too short and have cheap plated screws.

Here are the photos.

360kg of batterys almost make my Vitara pull wheel stands :)

Batteries bolted down

One set of 4 batteries connected. I need to buy some new bolts for the terminals to do the rest.

The battery balance circuits are attached to the 4 batteries in a set.

50 x balance systems.

Here are a few snaps of my battery balance system construction. they consist of 2 x terminals, 2 x 6.8v Zeners and one 4.7 Ohm resistor. Each series is encased in clear heatshrink.

Under construction. The wooden block is a jig for aligning the pieces for soldering

Completed units - no heatshrink

Completed units, with heatshrink.

A few notes

1. My batteries are on a ship on their way from China. Should arrive mid October.
2. Spent Saturday installing the dashboard and stereo. Gotta have good sounds!
3. I have located all the important wires in the loom and designed the necessary opto isolating interface. (Reverse, Tacho and Speedo)
4. I have permanently wired the brake vacuum pump to the ignition
5. Sent plans for battery box to Geoff for fabrication.
   

Cyclic test result for 20Ah Greensavers

I have cycle tested the Greensavers and plotted the capacity. The results were disappointing HOWEVER please see the explanation from Greensaver. It seems I over discharged them. I probably incorrectly assumed the 100% discharge point was indicated by the "Discharge Characteristic Curve" in the data sheet. (see previous post for data sheet)

Director of R&D, Greensaver: "Two comments on the cycle life test result: (a) The cutoff voltage was too low. Overdischarge will cause low cycle life. It is suggested that the depth of discharge is limited to 80%, and the cutoff voltage is set at 11.4 ~ 11.5V.

(b) The average discharge currents were relatively high. In this case, the recharge current is suggested to choose the same rate of high current to improve the cycle life of batteries."

Point 2 would be difficult to implement due to the cost, size and primary current of chargers.


The capacity is normalised to 1Ah discharge as varying discharge currents were used. This eliminates the effect of the Peukert factor at different discharge rates. The 'waves' are caused by slightly different end point volatages at different currents.

Batteries Ordered

Last week I ordered 55 x Greensaver SP20-12 batteries. 50 to install and 5 spares.

I need to design and build the battery box next!

Cost per battery should be a little under $60 each including landing taxes.

Greensaver battery inter cell balance - After Discharge

To satisfy Tuarns (and my) curiosity, I discharged the battery to 11.5v at 20A, let the battery rest, then measured the individual cell voltages. Following are the results.

+
2.0919
2.0966
2.0994
2.0985
2.1004
2.0957
-

Average = 2.097083
Standard Deviation = 0.003079
Range is from 2.0919 to 2.1004 difference = .0085 volts (0.406%)

Again the battery seems to be well balanced.

Greensaver inter cell balance

Greensaver have been very helpful in providing technical details regarding their batteries.
They provided the following diagram of the top of the cell including the interconnecting links.



I drilled 5 holes in the top of the battery according to their diagram and measured the voltage of the 6 individual cells. The battery is measured at full charge. This battery has been (unfortunately) over discharged about 35 times and is only about 1/2 new capacity. A battery that has been cycled like this should exhibit significant imbalance between cells if the cells are not almost identical.



The results below are measured with a meter (not pictured) with 4 decimal places:

Cell : Voltage

+
1: 2.2402v
2: 2.2456v
3: 2.2452v
4: 2.2541v
5: 2.2490v
6: 2.2458v
-

Average cell voltage is 2.2466 volts. This is 100% charged including some surface change.

The standard deviation is .004616

The range is from 2.2402 to 2.2541. A difference of 0.0139v (13.9mv) or 0.6205%

I am not an expert in battery chemistry but this seems to be quite good.

Comments welcome.

Battery Balancing the cheap way

I have installed a zener diode - resistor battery balancer on my 'self balancing scooter'

There are 2 x 6.8V diodes and a 5 ohm resistor across each battery.
(Thanks again for the info from Tuarn)

The Lead Acid batteries at rest are about 13.3V at new and fully charged so no power is lost when the batteries are not in use.

The Zener voltage must be greater than the battery full charge voltage

The idea is that the more voltage there is across a battery, the more current is bypassed that chargers lower batteries.

This worked surprisingly effectively.

The batteries before installing the balancer were from 12.5 to 14 volts at 'full' charge.
After balancing they were all within a few millivolts of each other, even after some discharge.

I was concerned that the bypass current may effect the automatic charger cutoff point as the batteries became fully charged however the charger still functions properly.

Given Zener voltage = 13.6
Maximum charger voltage = 15.2
Bypass resistor = 5 Ohms

The maximum current through the resistor is (15.2-13.6)/5 =.32A
The maximum resistor power dissipation is .32 * (15.2-13.6)=.5W

My ceramic 5W resistors are overkill. I will use 1W resistors in the vehicle system.

Maximum zener power dissipation will be 6.8V * .32A = 2.2W

The long awaited 20Ah Greensaver Test Results

Greensaver 20Ah Battery Testing

Apparatus:

Microprocessor controlled battery discharger and charger.

4 x 1 Ohm 200 watt resistors. Hard wire configurable.

Relay to switch between 2 discharge currents

Microprocessor calculates V, I, Temp, Ah, Wh and Peukert Factor.

Readings are taken every second and logged to the serial port of a computer for graphing.

After discharge, the battery is automatically charged.

The discharge cutoff voltage is programmable.

The negative figures in the LCD indicate that the battery is being charged.

I know, it looks like a weapon of mass destruction :)

Goal:

To determine the Peukert effect of the 20Ah and 27Ah Greensaver battery.

The reason for perform the test is because the specifications state a low Peukert factor fro these batteries and I wanted to verify this before purchasing a pack of 50 batteries.

Results:

Discharge curves. (note glitch in V5 between seconds 3 and 12. As this is only a 9 second occurrence, it does not affect the overall result.

Peukert Effect:

Conclusion:

The average Peukert factor of 1.105 is low compared to other batteries that are usually greater than 1.13 for batteries of similar capacity. The calculated Peukert factor from the 20Ah specification sheet is 1.07. My tester is probably not that accurate but is sufficient to confirm that the battery performs close to specification.

Given this result, if Lead acid batteries are used, they should have superior performance under load and give a long service life.

Original discharge data is available in an Excel sheet upon request.

Next steps:

I also have a 27Ah battery that will be tested next.

The high current wiring will be upgraded to allow testing at 40A

The discharger will also be reprogrammed to continuously charge and discharge a battery to determine de rating over time.

Charging:

The charge curve for the 20Ah battery is below.

Update - Batteries - Tester - Diff - Steering - VFD - Tray

It has been a few weeks since my last post so an update is in order. photos to follow.

1. I received the Greensaver batteries (20ah and 27ah - 2 hr rate). The port chargers were unbelievable! These would have to be the 2 most expensive batteries in Australia! Lesson learned about sea-freighting goods

2. I have built a micro controller based charger / discharger. It will measureand log to a PC the following aprameters during discharge and charge Volts, Amps, Watts, Ah, Wh, temperature and the all important Peukert factor. It uses 4 x 1 ohm resistors. The resistors can be wired differently to vary the test current. I am waiting an LCD from Futurelec and a charger from ebay's ecrazyman.

3. I have bought a diff from pick'n'payless . It is a 5.125 ratio from a Hiace van. All going well I will install it this weekend. This gives a theoretical top speed of 94km/hr at 4000RPM

4. I also bought a manual steering box from pick'n'payless for installing this weekend.

5. Better VFD. I bought a better Danfoss 5042 VFD off ebay. So, if anyone wants a Danfoss 6042VFD in an IP54 case I have one looking for a home. I have removed all the AC components from the cabinet to minimise losses and will feed it DC directly to the soft charge circuit.

6. The ute has a tray back which was way too wide for the body. To tidy up the looks I have narrowed the tray to be the same as the body (100mm off each side) and shortened the back. If I need sides I can get them from the manufacturer which is encouraging.

At the moment the carport looks like a car wreckers yard, but things should start coming together in the next few weeks. I am still waiting to borrow an engine hoist from my neighbours.

Greensaver batteries on order

I have ordered some sample Greensaver batteries. A 20AH, a 27Ah and a 12 volt charger.

I have also ordered 4 x 1 ohm 200w resistors for building a battery analyser. I am workon on the Bascom AVR code at the moment to check their capacity at different loads. The Peukert fact er of 1.09 for the 20Ah unit seems a little too good to be true but if it tests out after many cycles then they are great value for a bit over USD20.

Battery Balancer

I have been thinking about how best to balance my chain of batteries, all 50 of them.

As there are 50 units the price could be quite significant.

Ideally a microprocessor based system would be implemented with isolated sensors on each battery however at this time it is cost and time prohibitive.

There are 2 alternatives I have seen the first would place 15V Zener Diode in series with a light bulb or suitable resistor across the battery. When a battery exceeds 15 volts, the Zener and lamp would bypass current proportional to the over voltage. This would be the simplest to implement.

Another alternative is a more aggressive voltage clamp described here.

The clamp voltage will have to be above the maximum charge voltage from the chargers and the regenerative braking.

Under regen this is 15 volts. Under charge this is about 15 volts but will need to be measured on the charger.

Obviously this solution will only work under charge but should be sufficient for now.
I will test it on my self balancing scooter first.