Battery System Voltages and equivalent power

[Cariboocoot]

Because this is one of the issues that's asked about repeatedly (in different ways) I thought I'd just outline the basics here for reference.

For comparative purposes, this example uses eight "golf cart" batteries of 220 Amp hours and 6 Volts, or 1320 Watt hours (DC) each. Times eight that is a battery bank capacity of 10,560 Watt hours, up to 50% of which would be "usable" for 5,280wh.

We will also assume a 10% charge rate (the array sizes would be equal), 1200 Watt load, and standardization of other factors (including ignoring losses) because the point here is to relate the differences between the three normal system Voltages: 12, 24, and 48.

First, the 12 Volt system:
This would be four parallel strings of two batteries in series. That's 880 Amp hours @ 12 Volts.
Charge rate: 88 Amps, necessitating either a MidNite Classic controller or two other types of MPPT controllers to handle the current or at least 2 PWM controllers without the efficiency of going MPPT if the voltages are right for it.
The wiring should have positive and negative bus bars, individual battery string fuses, and twelve interconnecting wires.
The 1200 Watt load will draw 100 Amps.
Problems: keeping current flow even through all batteries, handling the charging current, many connections (points of failure).

Second, the 24 Volt system:
This would be two parallel strings of four batteries in series. That's 440 Amp hours @ 24 Volts.
Charge rate: 44 Amps, which can be handled by one charge controller of many different types/brands.
The wiring can be done with the "diagonal" system. Individual string fuses can be used, but not as critical as with the 12 Volt set-up. Only eight interconnecting wires.
The 1200 Watt load will draw 50 Amps.
Problems: most of the problems of the 12 Volt configuration are eliminated. The issue of keeping current flow even is still there, but greatly reduced. Far fewer connections.

Third, the 48 Volt system:
This would be one string of all eight batteries connected in series. That's 220 Amp hours @ 48 Volts.
Charge rate: 22 Amps. Easily handled by any number of different charge controllers.
No wiring difficulties; no need for individual string fuses. Only seven interconnecting wires.
The 1200 Watt load will draw 25 Amps.
Problems: minimal. Current flow will be as even through all batteries as possible. Fewest connections.

The point here is to demonstrate that the higher system Voltage eliminates many of the problems that come from a need to store and handle larger amounts of power (all the battery banks in the example have the same equivalent power capacity).

[Cariboocoot]

Second Part: Two sixes or two twelves?

Another Frequently Asked Question is whether it is better to use two 6 Volt batteries in series or two 12 Volt batteries in parallel on a 12 Volt system. Since this is related to the previous post (and can be derived from it if you analyze long enough) I've included it here.

For this example we will use two different "theoretical" batteries, as we only need to compare the configuration differences.

Two 6 Volt 200 Amp hour batteries in series:
200 Amp hours @ 12 Volts.
One battery interconnection.
Even current flow through both batteries.
Six cells to check.

Two 12 Volt 100 Amp hour batteries in parallel:
200 Amp hours @ 12 Volts.
Two battery interconnections.
Possible uneven current flow through both batteries, can be reduced by using proper wiring technique.
Twelve cells to check.

Of course the "cells to check" is dependent on them being flooded cell not AGM type, but it needs to be mentioned. Likewise using the diagonal wiring method (Smart Gauge method #2 http://www.smartgauge.co.uk/batt_con.html) virtually eliminates current sharing problems. But you can still see the difference between the two configurations.

[raydias]

Hello Cariboocoot

Could you add information on proper charge rates. Example if you have a 10 amps (12v) coming from your charge controller you wont get very far if you have a 500 amp (12v) battery bank. What's the proper % of the battery bank in charge capacity to properly charge a set size bank.

thanks

[Cariboocoot]

Hello Cariboocoot

Could you add information on proper charge rates. Example if you have a 10 amps (12v) coming from your charge controller you wont get very far if you have a 500 amp (12v) battery bank. What's the proper % of the battery bank in charge capacity to properly charge a set size bank.

thanks

The actual rate depends on the manufacturer's recommendations for the particular battery involved. For instance; as a rule flooded cell types have lower current limits than AGM's. But within any given battery type there are ranges and limits. Some of it has to do with the amount of time involved; a higher rate for a short time on occasion will not be as damaging as a consistently high (or low) charge rate.

For more information on battery charging, check the deep cycle battery FAQ's here: http://www.windsun.com/Batteries/Battery_FAQ.htm