The part of the project that arguably could make or break it. I have thought up two approaches to this issue:

Buy the cheapest off the shelf batteries in Costco and experiment – run a series of on road tests and get and idea of range / capacity in the real world. From here I would have useful stats about increasing power / range.

Buy high end batteries (Trojan look good) then put some real effort into getting the battery design right – and try to get right first time.

Option 1, Costco batteries

Costco have a 12v / 115Ah Marine / Deep cycle battery for $84 – trying to determine the OEM so I can run the numbers with evconvert calculator.

Useful web link: http://www.evconvert.com/tools/evcalc/?s=b

Using 12 batteries = $1008 + GA sales tax (6%)

Brought 2 x Costco batteries purchased for measurements / experimentation

Option 2, Using Trojan 6V

Requires 24 batteries @ $130 / each = $3,120

The most important consideration with the batteries is the potential range. I have settled on 144v in the previous discussion about controller and motor. Using 144v will drive using either 12 x 12v, 18 x 8v or 24 x 6v for the battery configuration.

Option 3, C&D Technologies (Dynasty UPS) – “Max Rate”

David offered option for a set of 12 second hand C&D Technologies AGM cell.

Battery specifications:

12v, AGM, 138Ah capacity

http://www.cdpowercom.com/product/battery/vrla/pdf/mr12_490.pdf

Getting their first charge…

Planned layout

battery-layout-B.pdf

State of charge calculations

Charge	Volts per battery	Pack voltage
100%	12.65+	152
75%	12.45	149
50%	12.24	147
25%	12.06	144
0%	11.85	142

Battery log / management system

Battery log – Manual measurements of battery voltages before and after various trips.

BMS – Design and work to build a automatic monitoring systems

Journal entries

2009 08 11 – Took delivery of 12 x C & D Technology, Dynasty Series, “Max Rate” AGM batteries

2009 08 12 – Rigged up a parallel charging harness and to allow a low rate charge (2A) using a standard automatic charger (on AGM setting.)

2009 10 19 – Battery racks in place

Reference designs

I started thinking about a faster method of monitoring the batteries, than the me with DVM on each battery. There are several commercial solutions out there, but they start in the $300 range for a basic system. For those kind of dollars some custom design options could be considered. Some Google research I found some existing projects that had build simple BMS systems for EVs:

http://home.earthlink.net/~evtkw/evbattmon.html

http://genki.home.ionet.net/BattMon/BattMonArticle.html

High level design

My idea was to try and build a simple monitoring device that would collect the data. Rather than build a custom module to be mounted in the card, I felt it would be nice to leverage other display devices I already own (~BlackBerry, iTouch, etc) which could be used for a variety of other purposes. The simple interface to such device is a basic web server and use the built in browser on the handheld device to display the data.

This approach has a number of advantages, it requires less engineering on the display side (just some basic web page design). It is also less expensive as the display device is always leveraged for other applications.

I found a basic PIC module for around $70 that has 12 ~ADCs, IP stack and even a web server demo application.

http://microcontrollershop.com/product_info.php?products_id=893&osCsid=cf78aef068dc74ad19a42afa585e5245

There is a mounting kit with a prototype card so that I can mount the interface on the prototype card and have the system nicely boxed up.

http://microcontrollershop.com/product_info.php?products_id=1389&osCsid=cf78aef068dc74ad19a42afa585e5245

Schematics

Following the generic interface design, I used the basic opto-isolater interface design to connect the battery to the PIC ADC inputs.

Calibration

I built that circuit up on a breadboard and carried some calibration tests

Measurements from test circuit

Vin	15	13.5	13	12	11	10	9	8	7	6
ADCin	1.05	1.50	1.64	1.95	2.26	2.57	2.88	3.19	3.51	3.82

Plot

Finding the slope: y = mx + b (m = -3.2491, b = 18.4115)

Convert to 8bit and 10bit for ADC maths

Vin	15	13.5	13	12	11	10	9	8	7	6
ADCin	1.05	1.50	1.64	1.95	2.26	2.57	2.88	3.19	3.51	3.82
8-bit val	54	77	84	100	116	132	147	163	180	196
10-bit val	215	307	336	399	463	526	590	653	719	782

8bit conversion formula:

BatV = -0.06338 * 8bADCval + 18.4115

10bit conversion formula:

BatV = -0.01587 * 10bADCval + 18.4115

Web interface

Prototype wiring

Battery log

Date	>	2/11	>	3/14	>	3/20	4/25	>	4/26	4/28	5/22
Distance	>	10mi	>	23.8mi	>	34mi		>	10mil
Battery	Start (v)	End (v)	End (v)	Charged (v)	End (v)	Charged (v)	Charged (v)	End (v)	Charged (v)	Charged (v)	Charged (v)
Pack	154.4	148.0	144.2	154.5	143.4	155.3	154.5	150.4	155.3	155.9	154.8
1	12.8	12.3	12.05	12.91	11.98	12.97	12.91	12.57	12.97	13.02	12.92
2	12.9	12.3	12.10	12.95	12.03	13.00	12.87	12.61	13.00	13.05	12.94
3	12.9	12.3	12.07	12.98	12.00	12.98	12.92	12.59	12.98	13.02	12.92
4	12.8	12.3	12.00	12.87	11.93	12.93	12.88	12.54	12.93	12.99	12.89
5	12.9	12.3	12.01	12.92	11.97	12.97	12.92	12.57	12.98	13.03	12.99
6	12.9	12.3	12.07	12.93	12.00	12.98	12.93	12.57	12.99	13.03	12.93
7	12.8	12.2	11.99	12.85	11.93	12.91	12.86	12.53	12.92	12.98	12.89
8	12.7	12.2	11.88	12.73	11.92	12.89	12.83	12.48	12.88	12.93	12.82
9	12.8	12.3	12.07	12.90	12.00	12.95	12.91	12.53	12.97	13.02	12.92
10	12.8	12.2	11.96	12.83	11.87	12.88	12.82	12.54	12.88	12.93	12.82
11	12.8	12.3	12.05	12.83	11.97	12.95	12.89	12.43	12.95	13.01	12.90
12	12.7	12.2	11.91	12.77	11.83	12.83	12.78	12.47	12.84	12.90	12.88

Watt/mile

Date	12/16	3/15	3/20	3/21	3/25	4/26	4/27	4/28
Distance (miles)	26	23.8	34	34	26	10	10	15
Charge (kWh)	12.2	11.62	13.9	11.7	11.59	4.18	4.3	6.2
Watts/mile	469	488	408	344	445	418	430	413

Battery layout

		6
7		5
8		4
9	front ->>
10
11		3
12		2
		1
Cargo door