Amp-hrs based charging control

Amp-hrs based charging control

In order to get the most from rechargeable batteries, they must be properly charged after each discharge.  This requirement is especially important for lead acid (flooded cell, AGM, Gel) chemistry batteries.  We explain the science behind this below.

In all types of battery chemistry, the electron flow that makes up the charge or discharge current is actually one of the components (‘ingredients’) of the electro-chemical reactions that occur in the cells.  Once we recognize this, it is quite easy to understand why one must return to the battery the same amount of charge (# of electrons, Amp-hrs) that was taken out during the previous discharge event.

Consider the chemical reaction that occurs in a lead acid cell, shown in a simplified manner below, where Pb is lead, H2SO4 is the electrolyte, PbO is lead oxide and PbSO4 is lead sulfate.

Negative plate discharge reaction (simplified):

Pb + H2SO4 -> PbSO4 + 2 electrons

Positive plate discharge reaction (simplified):

PbO + H2SO4 + 2 electrons -> PbSO4

The electrons required for the positive plate reaction is supplied by the reaction on the negative plate and this flow from the negative side to the positive side is what we see as the discharge current.

Both reactions produce lead sulfate (PbSO4) that gets deposited as thin layers on the plates.

During charging, the reactions proceed in the reverse direction.  Electrons supplied by an external source (solar, AC charger, etc) combine with lead sulfate in the presence of the electrolyte, and form lead and lead oxide on the negative and positive plates.

It is quite clear that the charging process would be incomplete if sufficient # of electrons are not supplied, leaving trace amounts of lead sulfate on the plates.

Over time, this layer of lead sulfate becomes a hard-to-breakdown layer that covers significant area of the plates, limiting the amount of reactants available to produce an electrical current.  This manifests as a permanent reduction in the battery’s capacity to hold charge and ability to deliver that charge as a strong electrical current.

It is not possible to prevent the creation of lead sulfate, since it an important component of the chemical reaction.  However, properly charging the batteries to ensure complete reversal each charging cycle will significantly reduce the amount of lead sulfate left behind and delay permanent loss of capacity due to sulfation.  Once formed, it is difficult to remove the sulfate layer, esp. in AGM & Gel type cells.

Bogart Engineering’s SC-2030, when used with a TM-2030, accurately measures the Amp-hrs returned to the battery during charging and compares it to the previous discharge Amp-hrs and sustains the charging process until the 100%  of the discharge Amp-hrs are returned.  It is also possible to configure the SC-2030 so that a specific amount of excess charge is delivered, to account for the inefficiencies in the charging process.  By implementing an Amp-hrs based charging on a new set of batteries, it is possible to extend battery life by 15-25%.

To our knowledge, the TM-2030/SC-2030 system is the only one in the market with Amp-hrs based charging termination feature.  Parameters specific to Amp-hrs based charging are set in P15 (voltage condition), P21 (current limit) and P20 (excess charge % relative to previous discharge).


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