Sunday, 29 January 2012

Assault and Battery (7)

Continued from Assault and Battery (6)

One Item that I missed was the effect on the boats power budget from small battery chargers. Most people have small battery pack chargers used for mobile phones. Often left sat in a socket waiting for the next charge. I was surprised to find that such devices consume power even when disconnected from the phone and in standby mode. It was the same with the television set. Remember to turn such items off. Otherwise whatever power is consumed on standby is a continuous draw on the battery bank.

Another question....

Q1) Why do some battery monitors have to be reset from time to time?

The voltage measured at the battery terminals can be used as a rough indication of the battery’s state of charge. However, if a more accurate figure is required then we need to monitor the battery much more carefully.

A battery monitoring system is the most accurate way to monitor a battery’s condition with regard to charge and available capacity. Some battery monitoring systems are quite simple and just measure the terminal voltage and any current (amps) flowing out of the battery. This only gives a reading for a specific moment in time. Other more expensive systems are capable of monitoring the number of amps flowing in (charging) and out (discharging) over time. In other words we are monitoring and recording the battery ampere hour performance.

So over time the battery monitor measures the charging and discharging cycle. Keeping a running total of what has gone in during the charge and what has been removed at discharge. The problem is that a battery's capacity to deliver amps changes with how much current is being drawn from the battery over time. If we have a fully charged 100ah battery and we draw 10 amps for four hours we should have in theory have a reserve of 60ah left. However if we discharge 40 amps for one hour we should have the same 60ah left in reserve. The reality is that at a higher discharge rate the capacity left will be much less and maybe as low as 30ah. Whilst after discharging at the lower 10 amp rate. The remaining capacity might be as much as 50ah. Another problem is that as batteries age, their capacity diminishes. So our 100ah battery when new, a year later may only hold 90ah when fully charged.

The reason for the reduced capacity at a high discharge rate is the rate of change of the chemical process inside the battery cannot keep up with the demand. This is highlighted when the terminal voltage goes low. A battery discharged at a very high rate which is then subjected to a lower rate of discharge can recover some of its lost capacity again over time. This is as the chemical process catches up again (called reforming) and can only occur where a reduced amount of power is being discharged. Conversely, a battery that is discharged at a slower rate of discharge will be able to convert over time much more of the chemical process into available capacity.

Now the monitoring problem becomes even more complex because we are unlikely to maintain a steady flow of amps out of a battery. As items switch on and off - such as a fridge, a water pump or any other intermittent  electrical item. The amount of discharge is going to vary over time. As we run the engine the rate of charge is going to change over time. A lead acid battery charging efficiency is poor. As the battery ages the efficiency will diminish even further. Our battery monitoring system will become more and more out of synchronisation over time.

When a battery is charged or discharged efficiency losses occur. We have to put more in, say as much as 30% and we get less capacity out which could be around 40% loss over the life of the battery. So there could be a 70% charging efficiency and a 60% discharging efficiency over time. But we don't need to monitor how efficient our batteries are unless they are nearing the end of their life.

So periodically we need to reset the monitoring system to a known condition. Opinion on how often this reset charge should happen is open to conjecture. I try to do mine about once a month for no other reason than I try to remember to do it at the start of each month.

The only way we can do that is to give the battery a complete recharge. However, when doing this full charge. It would be a good idea to go through the three main conditions of charging. Bulk charge, Absorption charge, Float charge which is sometimes confused with the Equalising charge.  
  • Bulk Charge is when the battery is charged at the full available charge current.
  • Absorption Charge is when charging is current limited to charge the battery at a slower rate to prevent the chemical process from gassing. The current going into the battery will steadily decrease as the battery reaches its fully charged state.
  • Float Charge is when the battery is maintained at a lower fixed voltage to compensate for any slight discharge.
  • Equalising Charge. Batteries will reach their fully equalised state, when float charged a long period of time. However, after your battery is fully charged, you can perform an equalising charge by allowing the battery charging voltage rise to 15.5v for three or four hours. The equalising charge allows individual cells in a battery that are not taking a full charge for some reason. An additional small charge time at an elevated voltage to recover to full charge capacity. This is not to be confused with overcharging a battery with a cheap automotive battery charger.
The more expensive battery monitors also have what is called "system event logging". They can record whenever certain types of events occur. Such as a full discharge of the battery. Or you may be able to set an alarm for when a certain monitored event occurs. Say a discharge below a fixed level. Or if current is being drawn out of the battery above a certain level.

So when I say a "reset" charge I don't mean a hard reset of the battery monitoring electronics. There is usually a combination of buttons to press to let the battery monitor know when the battery is fully charged. The battery monitor is then resynchronised to 100% capacity once more.

There are several battery monitoring systems available. From the very simple to the very complex. From the cheap and cheerful to the expensive techno toy!

On Rosie I use the victron BMV501 battery monitor. The data used by the BMV 501 is monitored to an accuracy better than 0,5 % and, employing Peukert’s formula, account is taken of the reduction of effective capacity of the battery when the discharge current increases. An optional temperature sensor helps to further improve accuracy where wide temperature variation is expected.

The standard display includes:
Battery voltage.
Battery charge/discharge current.
State of charge of the battery: in Ah or as a percentage.
Time to go: the time until the battery is flat at the current rate of discharge.
Battery temperature

The BMV 501 also records data of the performance and use of the battery:
Adjustable over and under voltage alarm settings.
Average depth of discharge.
The deepest discharge that occurred.
Number of charge/discharge cycles.
Number of times that the battery has been fully recharged.
Number of complete discharges.
Number of times that the under-voltage alarm has been triggered.
Number of times that the over-voltage alarm has been triggered.

A simple battery voltage monitor is the BEP Marine. This stylish, high-quality panel mount analogue voltmeter will help you to keep a close eye on your batteries and charging systems. Suitable for 12v systems only, it is colour coded for easy reference.

Another simple on is a Durite simple analogue voltmeter. Ideal for keeping an eye on your batteries and charging system. Features a backlight, suitable for vehicle or marine use and require a 53mm diameter hole for mounting. Available in 12V or 24V.

BM1 The BM-1 is designed to monitor the state of charge of lead acid battery systems with capacities of between 5 and 600 amp hours. With a simple, four-button operation, the unit continuously monitors voltage, the charging or discharging current, and the accumulated Ampere-hours taken from the battery bank. It also predicts the time remaining to full charge or discharge of the battery bank.

The Victron BMV-600S is Victrons newest high precision battery monitor. The essential function of a battery monitor is to calculate ampere-hours consumed and the state of charge of a battery. Ampere-hours consumed are calculated by integrating the current flowing in or out of the battery.

Xantrex LinkLITE battery monitor can measure currents up to 1,000Amps. It selectively displays voltage, charge and discharge current, consumed amorphous and remaining battery capacity. It is equipped with an internal programmable alarm relay, to run a generator when needed or to turn off devices when the battery voltage exceeds programmable boundaries. Description Defining the amount of energy available in a battery is a complex task since battery age, discharge current and temperature all influence the actual battery capacity. High performance measuring circuits, along with complex software algorithms, are used to exactly determine the remaining battery capacity.

SmartGauge battery monitor. Super simple installation, highly accurate state of charge meter. Cannot run out of synchronisation with the batteries. Volts display, Charge Status (percentage), user programmable alarm functions, interface to the SmartBank split charge control system.

The Sterling Power Management Panel is a combined instrument including four volt meters, four amp meters and one amp hour counter. All voltages, amps and amp hours can be monitored on a central LCD display. Because the current is measured via shunts, the heavy cables required for conventional in-line amp meters are not necessary. This ensures minimum cable usage, low voltage drops in the connections and less Radio Frequency Interference and Electromagnetic Interference around the instrument panel.

Previously Assault and Battery (1)
Previously Assault and Battery (2)
Previously Assault and Battery (3)
Previously Assault and Battery (4)
Previously Assault and Battery (5)
Previously Assault and Battery (6)


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