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smiler

Splitting a battery bank

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The description of what happens during charge and discharge is pretty much on all fours with the description in a 1940's specialist textbook on accumulator charging. At thst time it was common practuce to recharge accummulators from DC mains via a lamp board, the number and wattage of the lamps determining the charging current. Charging would continue untill all cells in a battery were gassing freely, meaning that all cells were able to attain a fully charged state in which even the sulphate in the  deepest layers of active material would get converted. It was considered safe to discharge down to 1.8V per cell: deeper discharge would result in the production of hard crystalline sulphate having a different crystal structure to the sulphate produced at higher discharge voltages, and it is the hard sulphate that is virtually impossible to convert back. The voltage drop at high current discharges is due to the sulphuric acid reacting with the plates by giving up its sulphur; the reaction generates water which dilutes the acid near the plates and hence affects the chemical reaction that generates the current. Allowing the battery to rest allows fresh concentrated acid to diffuse into the active material, thereby raising the voltage again.  

 

 I don't think that sort of constant current charging is practiced much these days. Charging until the voltage reaches a value indicative of full charge is fine if all cells are balanced. However, the voltage of a weak cell can rise to a higher level than the other cells, thereby tricking the control circuitry into terminating the charge cycle  before the other cells are fully charged.

 

Installing a solar charger is a good move. In a recent post of another thread the poster reported that his boat was fitted with solar panels last year, and that after several months of non-use, his batteries, which his instruments had indicated had lost a significant amount of their original capacity before the perid of non-use, were now being indicated as in good condition. Possibly the enforced period of resting had allowed the conversion of the deeper regions of the active material to be regenerated as there would have been plenty of time for the electrolyte to diffuse into the deepest sulphate layers, allowing them to become converted. 

 

Trickle  charging at a low current is known to be a good way of keeping a battery in good order, but is not practical in a canal boat whose battery is used every day. Rapid charging of really heavily discharged batteries is not recommended for ordinary batteries: the resulting high current density in the conductive regions of the plates between the higher resistance sulphated regions will give rise to differential thermal expansion between sulphated and non-sulphated regions, resulting in shedding of active plate material and loss of capacity.  Trickle charging keeps the current density and temperature rise low, reducing the likelihood of shedding active material.

Edited by Ronaldo47
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On 15/09/2020 at 18:11, smiler said:

Not trying to invent the wheel but.....

 

 

But it sounded like you were trying to reinvent the wheel.

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4 hours ago, blackrose said:

 

But it sounded like you were trying to reinvent the wheel.

Sorry - I should have proof reread my post betterer.

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15 hours ago, Ronaldo47 said:

The description of what happens during charge and discharge is pretty much on all fours with the description in a 1940's specialist textbook on accumulator charging. At thst time it was common practuce to recharge accummulators from DC mains via a lamp board, the number and wattage of the lamps determining the charging current. Charging would continue untill all cells in a battery were gassing freely, meaning that all cells were able to attain a fully charged state in which even the sulphate in the  deepest layers of active material would get converted. It was considered safe to discharge down to 1.8V per cell: deeper discharge would result in the production of hard crystalline sulphate having a different crystal structure to the sulphate produced at higher discharge voltages, and it is the hard sulphate that is virtually impossible to convert back. The voltage drop at high current discharges is due to the sulphuric acid reacting with the plates by giving up its sulphur; the reaction generates water which dilutes the acid near the plates and hence affects the chemical reaction that generates the current. Allowing the battery to rest allows fresh concentrated acid to diffuse into the active material, thereby raising the voltage again.  

 

 I don't think that sort of constant current charging is practiced much these days. Charging until the voltage reaches a value indicative of full charge is fine if all cells are balanced. However, the voltage of a weak cell can rise to a higher level than the other cells, thereby tricking the control circuitry into terminating the charge cycle  before the other cells are fully charged.

 

Installing a solar charger is a good move. In a recent post of another thread the poster reported that his boat was fitted with solar panels last year, and that after several months of non-use, his batteries, which his instruments had indicated had lost a significant amount of their original capacity before the perid of non-use, were now being indicated as in good condition. Possibly the enforced period of resting had allowed the conversion of the deeper regions of the active material to be regenerated as there would have been plenty of time for the electrolyte to diffuse into the deepest sulphate layers, allowing them to become converted. 

 

Trickle  charging at a low current is known to be a good way of keeping a battery in good order, but is not practical in a canal boat whose battery is used every day. Rapid charging of really heavily discharged batteries is not recommended for ordinary batteries: the resulting high current density in the conductive regions of the plates between the higher resistance sulphated regions will give rise to differential thermal expansion between sulphated and non-sulphated regions, resulting in shedding of active plate material and loss of capacity.  Trickle charging keeps the current density and temperature rise low, reducing the likelihood of shedding active material.

 

Constant current charging is exactly what BT used to do until the introduction of Valve Regulated Sealed Lead Acid cells in 1980.

 

The older open topped or glass encased cells used much more robust (and thus expensive) plate construction than those used in modern batteries, and constant current charging at C5 and terminating at 2.67 volts per cell would buckle the plates on modern batteries.

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